diff --git a/lib/mix/lib/mix/tasks/app.start.ex b/lib/mix/lib/mix/tasks/app.start.ex
index 727fec57940..9d802139c9d 100644
--- a/lib/mix/lib/mix/tasks/app.start.ex
+++ b/lib/mix/lib/mix/tasks/app.start.ex
@@ -30,6 +30,7 @@ defmodule Mix.Tasks.App.Start do
     * `--force` - forces compilation regardless of compilation times
     * `--temporary` - starts the application as temporary
     * `--permanent` - starts the application as permanent
+    * `--preload-modules` - preloads all modules defined in applications
     * `--no-compile` - does not compile even if files require compilation
     * `--no-protocols` - does not load consolidated protocols
     * `--no-archives-check` - does not check archives
@@ -38,11 +39,18 @@ defmodule Mix.Tasks.App.Start do
     * `--no-start` - does not start applications after compilation
 
   """
+
+  @switches [
+    permanent: :boolean,
+    temporary: :boolean,
+    preload_modules: :boolean
+  ]
+
   def run(args) do
     Mix.Project.get!()
     config = Mix.Project.config()
 
-    {opts, _, _} = OptionParser.parse(args, switches: [permanent: :boolean, temporary: :boolean])
+    {opts, _, _} = OptionParser.parse(args, switches: @switches)
     Mix.Task.run("loadpaths", args)
 
     unless "--no-compile" in args do
@@ -76,6 +84,13 @@ defmodule Mix.Tasks.App.Start do
       end
     else
       start(Mix.Project.config(), opts)
+
+      # If there is a build path, we will let the application
+      # that owns the build path do the actual check
+      unless config[:build_path] do
+        loaded = loaded_applications(opts)
+        check_configured(loaded)
+      end
     end
 
     :ok
@@ -97,13 +112,6 @@ defmodule Mix.Tasks.App.Start do
 
     type = type(config, opts)
     Enum.each(apps, &ensure_all_started(&1, type))
-
-    # If there is a build path, we will let the application
-    # that owns the build path do the actual check
-    unless config[:build_path] do
-      check_configured()
-    end
-
     :ok
   end
 
@@ -146,25 +154,35 @@ defmodule Mix.Tasks.App.Start do
     end
   end
 
-  defp check_configured() do
+  defp loaded_applications(opts) do
+    preload_modules? = opts[:preload_modules]
+
+    for {app, _, _} <- Application.loaded_applications() do
+      if modules = preload_modules? && Application.spec(app, :modules) do
+        :code.ensure_modules_loaded(modules)
+      end
+
+      app
+    end
+  end
+
+  defp check_configured(loaded) do
     configured = Mix.ProjectStack.configured_applications()
-    loaded = for {app, _, _} <- Application.loaded_applications(), do: app
 
-    _ =
-      for app <- configured -- loaded, :code.lib_dir(app) == {:error, :bad_name} do
-        Mix.shell().error("""
-        You have configured application #{inspect(app)} in your configuration
-        file, but the application is not available.
+    for app <- configured -- loaded, :code.lib_dir(app) == {:error, :bad_name} do
+      Mix.shell().error("""
+      You have configured application #{inspect(app)} in your configuration file,
+      but the application is not available.
 
-        This usually means one of:
+      This usually means one of:
 
         1. You have not added the application as a dependency in a mix.exs file.
 
         2. You are configuring an application that does not really exist.
 
-        Please ensure #{inspect(app)} exists or remove the configuration.
-        """)
-      end
+      Please ensure #{inspect(app)} exists or remove the configuration.
+      """)
+    end
 
     :ok
   end
diff --git a/lib/mix/lib/mix/tasks/run.ex b/lib/mix/lib/mix/tasks/run.ex
index a460d522e58..ca6e9298b6c 100644
--- a/lib/mix/lib/mix/tasks/run.ex
+++ b/lib/mix/lib/mix/tasks/run.ex
@@ -40,6 +40,7 @@ defmodule Mix.Tasks.Run do
     * `--eval`, `-e` - evaluate the given code
     * `--require`, `-r` - requires pattern before running the command
     * `--parallel`, `-p` - makes all requires parallel
+    * `--preload-modules` - preloads all modules defined in applications
     * `--no-compile` - does not compile even if files require compilation
     * `--no-deps-check` - does not check dependencies
     * `--no-archives-check` - does not check archives
@@ -67,7 +68,8 @@ defmodule Mix.Tasks.Run do
           start: :boolean,
           archives_check: :boolean,
           elixir_version_check: :boolean,
-          parallel_require: :keep
+          parallel_require: :keep,
+          preload_modules: :boolean
         ]
       )
 
diff --git a/lib/mix/lib/mix/tasks/test.ex b/lib/mix/lib/mix/tasks/test.ex
index 9c73e864d0e..d442f8ba42d 100644
--- a/lib/mix/lib/mix/tasks/test.ex
+++ b/lib/mix/lib/mix/tasks/test.ex
@@ -67,11 +67,12 @@ defmodule Mix.Tasks.Test do
     * `--no-elixir-version-check` - does not check the Elixir version from mix.exs
     * `--no-start` - does not start applications after compilation
     * `--only` - runs only tests that match the filter
+    * `--preload-modules` - preloads all modules defined in applications
     * `--raise` - raises if the test suite failed
     * `--seed` - seeds the random number generator used to randomize tests order;
       `--seed 0` disables randomization
-    * `--slowest` - prints timing information for the N slowest tests; automatically
-      sets `--trace`
+    * `--slowest` - prints timing information for the N slowest tests
+      Automatically sets `--trace` and `--preload-modules`
     * `--stale` - runs only tests which reference modules that changed since the
       last `test --stale`. You can read more about this option in the "Stale" section below.
     * `--timeout` - sets the timeout for the tests
@@ -190,7 +191,8 @@ defmodule Mix.Tasks.Test do
     stale: :boolean,
     listen_on_stdin: :boolean,
     formatter: :keep,
-    slowest: :integer
+    slowest: :integer,
+    preload_modules: :boolean
   ]
 
   @cover [output: "cover", tool: Cover]
@@ -235,7 +237,8 @@ defmodule Mix.Tasks.Test do
     # available in test_helper.exs. Then configure exunit again so
     # that command line options override test_helper.exs
     Mix.shell().print_app
-    Mix.Task.run("app.start", args)
+    app_start_args = if opts[:slowest], do: ["--preload-modules" | args], else: args
+    Mix.Task.run("app.start", app_start_args)
 
     # Ensure ExUnit is loaded.
     case Application.load(:ex_unit) do
diff --git a/lib/mix/test/mix/tasks/app.start_test.exs b/lib/mix/test/mix/tasks/app.start_test.exs
index 1cc51c16be5..403a2e53972 100644
--- a/lib/mix/test/mix/tasks/app.start_test.exs
+++ b/lib/mix/test/mix/tasks/app.start_test.exs
@@ -35,12 +35,18 @@ defmodule Mix.Tasks.App.StartTest do
       assert File.regular?("_build/dev/lib/app_start_sample/ebin/Elixir.A.beam")
       assert File.regular?("_build/dev/lib/app_start_sample/ebin/app_start_sample.app")
 
+      assert :code.is_loaded(A)
       refute List.keyfind(Application.started_applications(), :app_start_sample, 0)
       assert List.keyfind(Application.started_applications(), :logger, 0)
+      :code.delete(A)
 
       Mix.Tasks.App.Start.run([])
+      refute :code.is_loaded(A)
       assert List.keyfind(Application.started_applications(), :app_start_sample, 0)
       assert List.keyfind(Application.started_applications(), :logger, 0)
+
+      Mix.Tasks.App.Start.run(["--preload-modules"])
+      assert :code.is_loaded(A)
     end
   end