NetworkTime creates it's own frame time in 2019

Since we don't have double time there yet and the stopwatch will change during the frame which causes jitter and other artifacts
supersedes #3623
See also #2838

Assets/Mirror/Core/NetworkLoop.cs

@@ -186,7 +186,8 @@ static void NetworkEarlyUpdate()
             // loop functions run in edit mode and in play mode.
             // however, we only want to call NetworkServer/Client in play mode.
             if (!Application.isPlaying) return;
-
+
+            NetworkTime.EarlyUpdate();
             //Debug.Log($"NetworkEarlyUpdate {Time.time}");
             NetworkServer.NetworkEarlyUpdate();
             NetworkClient.NetworkEarlyUpdate();

Assets/Mirror/Core/NetworkTime.cs

@@ -54,7 +54,8 @@ public static double localTime
         //          it changes during the frame too.
         static readonly Stopwatch stopwatch = new Stopwatch();
         static NetworkTime() => stopwatch.Start();
-        public static double localTime => stopwatch.Elapsed.TotalSeconds;
+        static double localFrameTime = 0;
+        public static double localTime => localFrameTime;
 #endif
 
         /// <summary>The time in seconds since the server started.</summary>
@@ -231,5 +232,12 @@ internal static void OnServerPong(NetworkConnectionToClient conn, NetworkPongMes
             double newRtt = localTime - message.localTime;
             conn._rtt.Add(newRtt);
         }
+
+        internal static void EarlyUpdate()
+        {
+#if !UNITY_2020_3_OR_NEWER
+            localFrameTime = stopwatch.Elapsed.TotalSeconds;
+#endif
+        }
     }
 }