diff --git a/Project/Assets/ML-Agents/Examples/SharedAssets/Scripts/SensorBase.cs b/Project/Assets/ML-Agents/Examples/SharedAssets/Scripts/SensorBase.cs
index 86f5d46d21..eed6b7a282 100644
--- a/Project/Assets/ML-Agents/Examples/SharedAssets/Scripts/SensorBase.cs
+++ b/Project/Assets/ML-Agents/Examples/SharedAssets/Scripts/SensorBase.cs
@@ -33,7 +33,7 @@ public virtual int Write(ObservationWriter writer)
             float[] buffer = new float[numFloats];
             WriteObservation(buffer);
 
-            writer.AddRange(buffer);
+            writer.AddList(buffer);
 
             return numFloats;
         }
diff --git a/com.unity.ml-agents/CHANGELOG.md b/com.unity.ml-agents/CHANGELOG.md
index b74c70411b..236111ad25 100755
--- a/com.unity.ml-agents/CHANGELOG.md
+++ b/com.unity.ml-agents/CHANGELOG.md
@@ -23,6 +23,11 @@ removed when training with a player. The Editor still requires it to be clamped
 - Added the IHeuristicProvider interface to allow IActuators as well as Agent implement the Heuristic function to generate actions.
   Updated the Basic example and the Match3 Example to use Actuators.
   Changed the namespace and file names of classes in com.unity.ml-agents.extensions. (#4849)
+- Added `VectorSensor.AddObservation(IList<float>)`. `VectorSensor.AddObservation(IEnumerable<float>)`
+  is deprecated. The `IList` version is recommended, as it does not generate any
+  additional memory allocations. (#4887)
+- Added `ObservationWriter.AddList()` and deprecated `ObservationWriter.AddRange()`.
+  `AddList()` is recommended, as it does not generate any additional memory allocations. (#4887)
 
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 
@@ -30,6 +35,9 @@ removed when training with a player. The Editor still requires it to be clamped
 #### com.unity.ml-agents (C#)
 - Fix a compile warning about using an obsolete enum in `GrpcExtensions.cs`. (#4812)
 - CameraSensor now logs an error if the GraphicsDevice is null. (#4880)
+- Removed several memory allocations that happened during inference. On a test scene, this
+  reduced the amount of memory allocated by approximately 25%. (#4887)
+
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Fixed a bug that would cause an exception when `RunOptions` was deserialized via `pickle`. (#4842)
 - Fixed a bug that can cause a crash if a behavior can appear during training in multi-environment training. (#4872)
diff --git a/com.unity.ml-agents/Runtime/Inference/ApplierImpl.cs b/com.unity.ml-agents/Runtime/Inference/ApplierImpl.cs
index 6675545d4b..3952786306 100644
--- a/com.unity.ml-agents/Runtime/Inference/ApplierImpl.cs
+++ b/com.unity.ml-agents/Runtime/Inference/ApplierImpl.cs
@@ -21,12 +21,13 @@ public ContinuousActionOutputApplier(ActionSpec actionSpec)
             m_ActionSpec = actionSpec;
         }
 
-        public void Apply(TensorProxy tensorProxy, IEnumerable<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
+        public void Apply(TensorProxy tensorProxy, IList<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
         {
             var actionSize = tensorProxy.shape[tensorProxy.shape.Length - 1];
             var agentIndex = 0;
-            foreach (int agentId in actionIds)
+            for (var i = 0; i < actionIds.Count; i++)
             {
+                var agentId = actionIds[i];
                 if (lastActions.ContainsKey(agentId))
                 {
                     var actionBuffer = lastActions[agentId];
@@ -65,7 +66,7 @@ public DiscreteActionOutputApplier(ActionSpec actionSpec, int seed, ITensorAlloc
             m_ActionSpec = actionSpec;
         }
 
-        public void Apply(TensorProxy tensorProxy, IEnumerable<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
+        public void Apply(TensorProxy tensorProxy, IList<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
         {
             //var tensorDataProbabilities = tensorProxy.Data as float[,];
             var idActionPairList = actionIds as List<int> ?? actionIds.ToList();
@@ -109,9 +110,11 @@ public void Apply(TensorProxy tensorProxy, IEnumerable<int> actionIds, Dictionar
                 actionProbs.data.Dispose();
                 outputTensor.data.Dispose();
             }
+
             var agentIndex = 0;
-            foreach (int agentId in actionIds)
+            for (var i = 0; i < actionIds.Count; i++)
             {
+                var agentId = actionIds[i];
                 if (lastActions.ContainsKey(agentId))
                 {
                     var actionBuffer = lastActions[agentId];
@@ -209,12 +212,13 @@ public MemoryOutputApplier(
             m_Memories = memories;
         }
 
-        public void Apply(TensorProxy tensorProxy, IEnumerable<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
+        public void Apply(TensorProxy tensorProxy, IList<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
         {
             var agentIndex = 0;
             var memorySize = (int)tensorProxy.shape[tensorProxy.shape.Length - 1];
-            foreach (int agentId in actionIds)
+            for (var i = 0; i < actionIds.Count; i++)
             {
+                var agentId = actionIds[i];
                 List<float> memory;
                 if (!m_Memories.TryGetValue(agentId, out memory)
                     || memory.Count < memorySize)
@@ -246,13 +250,14 @@ public BarracudaMemoryOutputApplier(
             m_Memories = memories;
         }
 
-        public void Apply(TensorProxy tensorProxy, IEnumerable<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
+        public void Apply(TensorProxy tensorProxy, IList<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
         {
             var agentIndex = 0;
             var memorySize = (int)tensorProxy.shape[tensorProxy.shape.Length - 1];
 
-            foreach (int agentId in actionIds)
+            for (var i = 0; i < actionIds.Count; i++)
             {
+                var agentId = actionIds[i];
                 List<float> memory;
                 if (!m_Memories.TryGetValue(agentId, out memory)
                     || memory.Count < memorySize * m_MemoriesCount)
diff --git a/com.unity.ml-agents/Runtime/Inference/GeneratorImpl.cs b/com.unity.ml-agents/Runtime/Inference/GeneratorImpl.cs
index 40853ec8ea..25c65cb87b 100644
--- a/com.unity.ml-agents/Runtime/Inference/GeneratorImpl.cs
+++ b/com.unity.ml-agents/Runtime/Inference/GeneratorImpl.cs
@@ -21,7 +21,7 @@ public BiDimensionalOutputGenerator(ITensorAllocator allocator)
             m_Allocator = allocator;
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             TensorUtils.ResizeTensor(tensorProxy, batchSize, m_Allocator);
         }
@@ -40,7 +40,7 @@ public BatchSizeGenerator(ITensorAllocator allocator)
             m_Allocator = allocator;
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             tensorProxy.data?.Dispose();
             tensorProxy.data = m_Allocator.Alloc(new TensorShape(1, 1));
@@ -63,7 +63,7 @@ public SequenceLengthGenerator(ITensorAllocator allocator)
             m_Allocator = allocator;
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             tensorProxy.shape = new long[0];
             tensorProxy.data?.Dispose();
@@ -92,14 +92,15 @@ public RecurrentInputGenerator(
         }
 
         public void Generate(
-            TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+            TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             TensorUtils.ResizeTensor(tensorProxy, batchSize, m_Allocator);
 
             var memorySize = tensorProxy.shape[tensorProxy.shape.Length - 1];
             var agentIndex = 0;
-            foreach (var infoSensorPair in infos)
+            for (var infoIndex = 0; infoIndex < infos.Count; infoIndex++)
             {
+                var infoSensorPair = infos[infoIndex];
                 var info = infoSensorPair.agentInfo;
                 List<float> memory;
 
@@ -147,14 +148,15 @@ public BarracudaRecurrentInputGenerator(
             m_Memories = memories;
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             TensorUtils.ResizeTensor(tensorProxy, batchSize, m_Allocator);
 
             var memorySize = (int)tensorProxy.shape[tensorProxy.shape.Length - 1];
             var agentIndex = 0;
-            foreach (var infoSensorPair in infos)
+            for (var infoIndex = 0; infoIndex < infos.Count; infoIndex++)
             {
+                var infoSensorPair = infos[infoIndex];
                 var info = infoSensorPair.agentInfo;
                 var offset = memorySize * m_MemoryIndex;
                 List<float> memory;
@@ -200,14 +202,15 @@ public PreviousActionInputGenerator(ITensorAllocator allocator)
             m_Allocator = allocator;
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             TensorUtils.ResizeTensor(tensorProxy, batchSize, m_Allocator);
 
             var actionSize = tensorProxy.shape[tensorProxy.shape.Length - 1];
             var agentIndex = 0;
-            foreach (var infoSensorPair in infos)
+            for (var infoIndex = 0; infoIndex < infos.Count; infoIndex++)
             {
+                var infoSensorPair = infos[infoIndex];
                 var info = infoSensorPair.agentInfo;
                 var pastAction = info.storedActions.DiscreteActions;
                 if (!pastAction.IsEmpty())
@@ -238,14 +241,15 @@ public ActionMaskInputGenerator(ITensorAllocator allocator)
             m_Allocator = allocator;
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             TensorUtils.ResizeTensor(tensorProxy, batchSize, m_Allocator);
 
             var maskSize = tensorProxy.shape[tensorProxy.shape.Length - 1];
             var agentIndex = 0;
-            foreach (var infoSensorPair in infos)
+            for (var infoIndex = 0; infoIndex < infos.Count; infoIndex++)
             {
+                var infoSensorPair = infos[infoIndex];
                 var agentInfo = infoSensorPair.agentInfo;
                 var maskList = agentInfo.discreteActionMasks;
                 for (var j = 0; j < maskSize; j++)
@@ -274,7 +278,7 @@ public RandomNormalInputGenerator(int seed, ITensorAllocator allocator)
             m_Allocator = allocator;
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             TensorUtils.ResizeTensor(tensorProxy, batchSize, m_Allocator);
             TensorUtils.FillTensorWithRandomNormal(tensorProxy, m_RandomNormal);
@@ -303,12 +307,13 @@ public void AddSensorIndex(int sensorIndex)
             m_SensorIndices.Add(sensorIndex);
         }
 
-        public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos)
+        public void Generate(TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos)
         {
             TensorUtils.ResizeTensor(tensorProxy, batchSize, m_Allocator);
             var agentIndex = 0;
-            foreach (var info in infos)
+            for (var infoIndex = 0; infoIndex < infos.Count; infoIndex++)
             {
+                var info = infos[infoIndex];
                 if (info.agentInfo.done)
                 {
                     // If the agent is done, we might have a stale reference to the sensors
@@ -320,8 +325,9 @@ public void Generate(TensorProxy tensorProxy, int batchSize, IEnumerable<AgentIn
                 {
                     var tensorOffset = 0;
                     // Write each sensor consecutively to the tensor
-                    foreach (var sensorIndex in m_SensorIndices)
+                    for (var sensorIndexIndex = 0; sensorIndexIndex < m_SensorIndices.Count; sensorIndexIndex++)
                     {
+                        var sensorIndex = m_SensorIndices[sensorIndexIndex];
                         var sensor = info.sensors[sensorIndex];
                         m_ObservationWriter.SetTarget(tensorProxy, agentIndex, tensorOffset);
                         var numWritten = sensor.Write(m_ObservationWriter);
diff --git a/com.unity.ml-agents/Runtime/Inference/ModelRunner.cs b/com.unity.ml-agents/Runtime/Inference/ModelRunner.cs
index ef4fda6a3d..7143b273cc 100644
--- a/com.unity.ml-agents/Runtime/Inference/ModelRunner.cs
+++ b/com.unity.ml-agents/Runtime/Inference/ModelRunner.cs
@@ -24,12 +24,14 @@ internal class ModelRunner
         TensorApplier m_TensorApplier;
 
         NNModel m_Model;
+        string m_ModelName;
         InferenceDevice m_InferenceDevice;
         IWorker m_Engine;
         bool m_Verbose = false;
         string[] m_OutputNames;
         IReadOnlyList<TensorProxy> m_InferenceInputs;
-        IReadOnlyList<TensorProxy> m_InferenceOutputs;
+        List<TensorProxy> m_InferenceOutputs;
+        Dictionary<string, Tensor> m_InputsByName;
         Dictionary<int, List<float>> m_Memories = new Dictionary<int, List<float>>();
 
         SensorShapeValidator m_SensorShapeValidator = new SensorShapeValidator();
@@ -56,6 +58,7 @@ public ModelRunner(
         {
             Model barracudaModel;
             m_Model = model;
+            m_ModelName = model.name;
             m_InferenceDevice = inferenceDevice;
             m_TensorAllocator = new TensorCachingAllocator();
             if (model != null)
@@ -84,6 +87,8 @@ public ModelRunner(
                 seed, m_TensorAllocator, m_Memories, barracudaModel);
             m_TensorApplier = new TensorApplier(
                 actionSpec, seed, m_TensorAllocator, m_Memories, barracudaModel);
+            m_InputsByName = new Dictionary<string, Tensor>();
+            m_InferenceOutputs = new List<TensorProxy>();
         }
 
         public InferenceDevice InferenceDevice
@@ -96,15 +101,14 @@ public NNModel Model
             get { return m_Model; }
         }
 
-        static Dictionary<string, Tensor> PrepareBarracudaInputs(IEnumerable<TensorProxy> infInputs)
+        void PrepareBarracudaInputs(IReadOnlyList<TensorProxy> infInputs)
         {
-            var inputs = new Dictionary<string, Tensor>();
-            foreach (var inp in infInputs)
+            m_InputsByName.Clear();
+            for (var i = 0; i < infInputs.Count; i++)
             {
-                inputs[inp.name] = inp.data;
+                var inp = infInputs[i];
+                m_InputsByName[inp.name] = inp.data;
             }
-
-            return inputs;
         }
 
         public void Dispose()
@@ -114,16 +118,14 @@ public void Dispose()
             m_TensorAllocator?.Reset(false);
         }
 
-        List<TensorProxy> FetchBarracudaOutputs(string[] names)
+        void FetchBarracudaOutputs(string[] names)
         {
-            var outputs = new List<TensorProxy>();
+            m_InferenceOutputs.Clear();
             foreach (var n in names)
             {
                 var output = m_Engine.PeekOutput(n);
-                outputs.Add(TensorUtils.TensorProxyFromBarracuda(output, n));
+                m_InferenceOutputs.Add(TensorUtils.TensorProxyFromBarracuda(output, n));
             }
-
-            return outputs;
         }
 
         public void PutObservations(AgentInfo info, List<ISensor> sensors)
@@ -169,31 +171,33 @@ public void DecideBatch()
             }
 
             Profiler.BeginSample("ModelRunner.DecideAction");
+            Profiler.BeginSample(m_ModelName);
 
-            Profiler.BeginSample($"MLAgents.{m_Model.name}.GenerateTensors");
+            Profiler.BeginSample($"GenerateTensors");
             // Prepare the input tensors to be feed into the engine
             m_TensorGenerator.GenerateTensors(m_InferenceInputs, currentBatchSize, m_Infos);
             Profiler.EndSample();
 
-            Profiler.BeginSample($"MLAgents.{m_Model.name}.PrepareBarracudaInputs");
-            var inputs = PrepareBarracudaInputs(m_InferenceInputs);
+            Profiler.BeginSample($"PrepareBarracudaInputs");
+            PrepareBarracudaInputs(m_InferenceInputs);
             Profiler.EndSample();
 
             // Execute the Model
-            Profiler.BeginSample($"MLAgents.{m_Model.name}.ExecuteGraph");
-            m_Engine.Execute(inputs);
+            Profiler.BeginSample($"ExecuteGraph");
+            m_Engine.Execute(m_InputsByName);
             Profiler.EndSample();
 
-            Profiler.BeginSample($"MLAgents.{m_Model.name}.FetchBarracudaOutputs");
-            m_InferenceOutputs = FetchBarracudaOutputs(m_OutputNames);
+            Profiler.BeginSample($"FetchBarracudaOutputs");
+            FetchBarracudaOutputs(m_OutputNames);
             Profiler.EndSample();
 
-            Profiler.BeginSample($"MLAgents.{m_Model.name}.ApplyTensors");
+            Profiler.BeginSample($"ApplyTensors");
             // Update the outputs
             m_TensorApplier.ApplyTensors(m_InferenceOutputs, m_OrderedAgentsRequestingDecisions, m_LastActionsReceived);
             Profiler.EndSample();
 
-            Profiler.EndSample();
+            Profiler.EndSample(); // end name
+            Profiler.EndSample(); // end ModelRunner.DecideAction
 
             m_Infos.Clear();
 
diff --git a/com.unity.ml-agents/Runtime/Inference/TensorApplier.cs b/com.unity.ml-agents/Runtime/Inference/TensorApplier.cs
index 72b2eca84d..d3d2d393f7 100644
--- a/com.unity.ml-agents/Runtime/Inference/TensorApplier.cs
+++ b/com.unity.ml-agents/Runtime/Inference/TensorApplier.cs
@@ -31,7 +31,7 @@ public interface IApplier
             /// </param>
             /// <param name="actionIds"> List of Agents Ids that will be updated using the tensor's data</param>
             /// <param name="lastActions"> Dictionary of AgentId to Actions to be updated</param>
-            void Apply(TensorProxy tensorProxy, IEnumerable<int> actionIds, Dictionary<int, ActionBuffers> lastActions);
+            void Apply(TensorProxy tensorProxy, IList<int> actionIds, Dictionary<int, ActionBuffers> lastActions);
         }
 
         readonly Dictionary<string, IApplier> m_Dict = new Dictionary<string, IApplier>();
@@ -90,10 +90,11 @@ public TensorApplier(
         /// <exception cref="UnityAgentsException"> One of the tensor does not have an
         /// associated applier.</exception>
         public void ApplyTensors(
-            IEnumerable<TensorProxy> tensors, IEnumerable<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
+            IReadOnlyList<TensorProxy> tensors, IList<int> actionIds, Dictionary<int, ActionBuffers> lastActions)
         {
-            foreach (var tensor in tensors)
+            for (var tensorIndex = 0; tensorIndex < tensors.Count; tensorIndex++)
             {
+                var tensor = tensors[tensorIndex];
                 if (!m_Dict.ContainsKey(tensor.name))
                 {
                     throw new UnityAgentsException(
diff --git a/com.unity.ml-agents/Runtime/Inference/TensorGenerator.cs b/com.unity.ml-agents/Runtime/Inference/TensorGenerator.cs
index b8650b6c01..02194ca6b5 100644
--- a/com.unity.ml-agents/Runtime/Inference/TensorGenerator.cs
+++ b/com.unity.ml-agents/Runtime/Inference/TensorGenerator.cs
@@ -31,7 +31,7 @@ public interface IGenerator
             /// the tensor's data.
             /// </param>
             void Generate(
-                TensorProxy tensorProxy, int batchSize, IEnumerable<AgentInfoSensorsPair> infos);
+                TensorProxy tensorProxy, int batchSize, IList<AgentInfoSensorsPair> infos);
         }
 
         readonly Dictionary<string, IGenerator> m_Dict = new Dictionary<string, IGenerator>();
@@ -149,10 +149,11 @@ public void InitializeObservations(List<ISensor> sensors, ITensorAllocator alloc
         /// <exception cref="UnityAgentsException"> One of the tensor does not have an
         /// associated generator.</exception>
         public void GenerateTensors(
-            IEnumerable<TensorProxy> tensors, int currentBatchSize, IEnumerable<AgentInfoSensorsPair> infos)
+            IReadOnlyList<TensorProxy> tensors, int currentBatchSize, IList<AgentInfoSensorsPair> infos)
         {
-            foreach (var tensor in tensors)
+            for (var tensorIndex = 0; tensorIndex < tensors.Count; tensorIndex++)
             {
+                var tensor = tensors[tensorIndex];
                 if (!m_Dict.ContainsKey(tensor.name))
                 {
                     throw new UnityAgentsException(
diff --git a/com.unity.ml-agents/Runtime/Sensors/ObservationWriter.cs b/com.unity.ml-agents/Runtime/Sensors/ObservationWriter.cs
index c27dc79a94..bbdb4e4622 100644
--- a/com.unity.ml-agents/Runtime/Sensors/ObservationWriter.cs
+++ b/com.unity.ml-agents/Runtime/Sensors/ObservationWriter.cs
@@ -64,7 +64,7 @@ internal void SetTarget(TensorProxy tensorProxy, int batchIndex, int channelOffs
         }
 
         /// <summary>
-        /// 1D write access at a specified index. Use AddRange if possible instead.
+        /// 1D write access at a specified index. Use AddList if possible instead.
         /// </summary>
         /// <param name="index">Index to write to.</param>
         public float this[int index]
@@ -122,6 +122,7 @@ public float this[int index]
         /// </summary>
         /// <param name="data"></param>
         /// <param name="writeOffset">Optional write offset.</param>
+        [Obsolete("Use AddList() for better performance")]
         public void AddRange(IEnumerable<float> data, int writeOffset = 0)
         {
             if (m_Data != null)
@@ -144,6 +145,27 @@ public void AddRange(IEnumerable<float> data, int writeOffset = 0)
             }
         }
 
+        public void AddList(IList<float> data, int writeOffset = 0)
+        {
+            if (m_Data != null)
+            {
+                for (var index = 0; index < data.Count; index++)
+                {
+                    var val = data[index];
+                    m_Data[index + m_Offset + writeOffset] = val;
+
+                }
+            }
+            else
+            {
+                for (var index = 0; index < data.Count; index++)
+                {
+                    var val = data[index];
+                    m_Proxy.data[m_Batch, index + m_Offset + writeOffset] = val;
+                }
+            }
+        }
+
         /// <summary>
         /// Write the Vector3 components.
         /// </summary>
diff --git a/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs b/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs
index 1f84b7c520..2b0b62cfca 100644
--- a/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs
+++ b/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs
@@ -329,7 +329,7 @@ public int Write(ObservationWriter writer)
                     rayOutput.ToFloatArray(numDetectableTags, rayIndex, m_Observations);
                 }
                 // Finally, add the observations to the ObservationWriter
-                writer.AddRange(m_Observations);
+                writer.AddList(m_Observations);
             }
             return m_Observations.Length;
         }
diff --git a/com.unity.ml-agents/Runtime/Sensors/StackingSensor.cs b/com.unity.ml-agents/Runtime/Sensors/StackingSensor.cs
index b9d55f6060..80c914c3ea 100644
--- a/com.unity.ml-agents/Runtime/Sensors/StackingSensor.cs
+++ b/com.unity.ml-agents/Runtime/Sensors/StackingSensor.cs
@@ -112,7 +112,7 @@ public int Write(ObservationWriter writer)
                 for (var i = 0; i < m_NumStackedObservations; i++)
                 {
                     var obsIndex = (m_CurrentIndex + 1 + i) % m_NumStackedObservations;
-                    writer.AddRange(m_StackedObservations[obsIndex], numWritten);
+                    writer.AddList(m_StackedObservations[obsIndex], numWritten);
                     numWritten += m_UnstackedObservationSize;
                 }
             }
diff --git a/com.unity.ml-agents/Runtime/Sensors/VectorSensor.cs b/com.unity.ml-agents/Runtime/Sensors/VectorSensor.cs
index 6c60c5c81c..d5e028c01d 100644
--- a/com.unity.ml-agents/Runtime/Sensors/VectorSensor.cs
+++ b/com.unity.ml-agents/Runtime/Sensors/VectorSensor.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using UnityEngine;
@@ -57,7 +58,7 @@ public int Write(ObservationWriter writer)
                     m_Observations.Add(0);
                 }
             }
-            writer.AddRange(m_Observations);
+            writer.AddList(m_Observations);
             return expectedObservations;
         }
 
@@ -164,6 +165,7 @@ public void AddObservation(Vector2 observation)
         /// Adds a collection of float observations to the vector observations of the agent.
         /// </summary>
         /// <param name="observation">Observation.</param>
+        [Obsolete("Use AddObservation(IList<float>) for better performance.")]
         public void AddObservation(IEnumerable<float> observation)
         {
             foreach (var f in observation)
@@ -172,6 +174,18 @@ public void AddObservation(IEnumerable<float> observation)
             }
         }
 
+        /// <summary>
+        /// Adds a list or array of float observations to the vector observations of the agent.
+        /// </summary>
+        /// <param name="observation">Observation.</param>
+        public void AddObservation(IList<float> observation)
+        {
+            for (var i = 0; i < observation.Count; i++)
+            {
+                AddFloatObs(observation[i]);
+            }
+        }
+
         /// <summary>
         /// Adds a quaternion observation to the vector observations of the agent.
         /// </summary>
diff --git a/com.unity.ml-agents/Tests/Editor/Sensor/ObservationWriterTests.cs b/com.unity.ml-agents/Tests/Editor/Sensor/ObservationWriterTests.cs
index 859d26d6a8..813c716a88 100644
--- a/com.unity.ml-agents/Tests/Editor/Sensor/ObservationWriterTests.cs
+++ b/com.unity.ml-agents/Tests/Editor/Sensor/ObservationWriterTests.cs
@@ -26,14 +26,14 @@ public void TestWritesToIList()
             writer[0] = 3f;
             Assert.AreEqual(new[] { 1f, 3f, 2f }, buffer);
 
-            // AddRange
+            // AddList
             writer.SetTarget(buffer, shape, 0);
-            writer.AddRange(new[] { 4f, 5f });
+            writer.AddList(new[] { 4f, 5f });
             Assert.AreEqual(new[] { 4f, 5f, 2f }, buffer);
 
-            // AddRange with offset
+            // AddList with offset
             writer.SetTarget(buffer, shape, 1);
-            writer.AddRange(new[] { 6f, 7f });
+            writer.AddList(new[] { 6f, 7f });
             Assert.AreEqual(new[] { 4f, 6f, 7f }, buffer);
         }
 
@@ -60,7 +60,7 @@ public void TestWritesToTensor()
             Assert.AreEqual(2f, t.data[1, 1]);
             Assert.AreEqual(3f, t.data[1, 2]);
 
-            // AddRange
+            // AddList
             t = new TensorProxy
             {
                 valueType = TensorProxy.TensorType.FloatingPoint,
@@ -68,7 +68,7 @@ public void TestWritesToTensor()
             };
 
             writer.SetTarget(t, 1, 1);
-            writer.AddRange(new[] { -1f, -2f });
+            writer.AddList(new[] { -1f, -2f });
             Assert.AreEqual(0f, t.data[0, 0]);
             Assert.AreEqual(0f, t.data[0, 1]);
             Assert.AreEqual(0f, t.data[0, 2]);
diff --git a/docs/Migrating.md b/docs/Migrating.md
index dd0d6f77e2..0eadcb9f06 100644
--- a/docs/Migrating.md
+++ b/docs/Migrating.md
@@ -17,7 +17,10 @@ double-check that the versions are in the same. The versions can be found in
 ## Migrating to Release 13
 ### Implementing IHeuristic in your IActuator implementations
  - If you have any custom actuators, you can now implement the `IHeuristicProvider` interface to have your actuator
-handle the generation of actions when an Agent is running in heuristic mode.
+  handle the generation of actions when an Agent is running in heuristic mode.
+- `VectorSensor.AddObservation(IEnumerable<float>)` is deprecated. Use `VectorSensor.AddObservation(IList<float>)`
+  instead.
+- `ObservationWriter.AddRange()` is deprecated. Use `ObservationWriter.AddList()` instead.
 
 
 # Migrating