diff --git a/src/fsharp/ParseAndCheckInputs.fs b/src/fsharp/ParseAndCheckInputs.fs
index 5ad43a53542..cb650d9cbda 100644
--- a/src/fsharp/ParseAndCheckInputs.fs
+++ b/src/fsharp/ParseAndCheckInputs.fs
@@ -769,7 +769,10 @@ type TcState =
         { x with tcsTcSigEnv = tcEnvAtEndOfLastInput
                  tcsTcImplEnv = tcEnvAtEndOfLastInput }
 
+    member x.RemoveImpl qualifiedNameOfFile =
+        { x with tcsRootImpls = x.tcsRootImpls.Remove(qualifiedNameOfFile) }
 
+ 
 /// Create the initial type checking state for compiling an assembly
 let GetInitialTcState(m, ccuName, tcConfig: TcConfig, tcGlobals, tcImports: TcImports, niceNameGen, tcEnv0, openDecls0) =
     ignore tcImports
@@ -946,15 +949,23 @@ let TypeCheckOneInput(checkForErrors,
     }
 
 /// Typecheck a single file (or interactive entry into F# Interactive)
-let TypeCheckOneInputEntry (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt) tcState inp =
+let TypeCheckOneInputEntryAux (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt) tcState inp skipImplIfSigExists =
     // 'use' ensures that the warning handler is restored at the end
     use unwindEL = PushErrorLoggerPhaseUntilUnwind(fun oldLogger -> GetErrorLoggerFilteringByScopedPragmas(false, GetScopedPragmasForInput inp, oldLogger) )
     use unwindBP = PushThreadBuildPhaseUntilUnwind BuildPhase.TypeCheck
 
     RequireCompilationThread ctok
-    TypeCheckOneInput (checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt, TcResultsSink.NoSink, tcState, inp, false)
+    TypeCheckOneInput (checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt, TcResultsSink.NoSink, tcState, inp, skipImplIfSigExists)
         |> Cancellable.runWithoutCancellation
 
+/// Typecheck a single file (or interactive entry into F# Interactive)
+let TypeCheckOneInputEntry (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt) tcState inp =
+    TypeCheckOneInputEntryAux(ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt) tcState inp false
+
+/// Typecheck a single file but skip it if the file is an impl and has a backing sig
+let TypeCheckOneInputEntrySkipImpl (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt) tcState inp =
+    TypeCheckOneInputEntryAux(ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt) tcState inp true
+
 /// Finish checking multiple files (or one interactive entry into F# Interactive)
 let TypeCheckMultipleInputsFinish(results, tcState: TcState) =
     let tcEnvsAtEndFile, topAttrs, implFiles, ccuSigsForFiles = List.unzip4 results
@@ -983,10 +994,40 @@ let TypeCheckClosedInputSetFinish (declaredImpls: TypedImplFile list, tcState) =
         errorR(Error(FSComp.SR.buildSignatureWithoutImplementation(qualNameOfFile.Text), qualNameOfFile.Range)))
 
     tcState, declaredImpls, ccuContents
+    
+let TypeCheckClosedInputSet (ctok, checkForErrors, tcConfig: TcConfig, tcImports, tcGlobals, prefixPathOpt, tcState, inputs) =
+    // tcEnvAtEndOfLastFile is the environment required by fsi.exe when incrementally adding definitions 
+    let results, tcState =
+        if tcConfig.concurrentBuild then
+            let results, tcState = (tcState, inputs) ||> List.mapFold (TypeCheckOneInputEntrySkipImpl (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt)) 
+
+            let inputs = Array.ofList inputs
+            let newResults = Array.ofList results
+            let results = Array.ofList results
+
+            (inputs, results) 
+            ||> Array.zip
+            |> Array.mapi (fun i (input, (_, _, implOpt, _)) ->
+                match implOpt with
+                | None -> None
+                | Some impl ->
+                    match impl with
+                    | TypedImplFile.TImplFile(qualifiedNameOfFile=qualifiedNameOfFile;implementationExpressionWithSignature=ModuleOrNamespaceExprWithSig.ModuleOrNamespaceExprWithSig(contents=ModuleOrNamespaceExpr.TMDefs [])) ->
+                        Some(i, input, qualifiedNameOfFile)
+                    | _ ->
+                        None
+            )
+            |> Array.choose id
+            |> ArrayParallel.iter (fun (i, input, qualifiedNameOfFile) ->
+                let tcState = tcState.RemoveImpl(qualifiedNameOfFile)
+                let result, _ = TypeCheckOneInputEntry (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt) tcState input
+                newResults.[i] <- result
+            )
+
+            newResults |> List.ofArray, tcState
+        else
+            (tcState, inputs) ||> List.mapFold (TypeCheckOneInputEntry (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt)) 
 
-let TypeCheckClosedInputSet (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt, tcState, inputs) =
-    // tcEnvAtEndOfLastFile is the environment required by fsi.exe when incrementally adding definitions
-    let results, tcState = (tcState, inputs) ||> List.mapFold (TypeCheckOneInputEntry (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt))
     let (tcEnvAtEndOfLastFile, topAttrs, implFiles, _), tcState = TypeCheckMultipleInputsFinish(results, tcState)
     let tcState, declaredImpls, ccuContents = TypeCheckClosedInputSetFinish (implFiles, tcState)
     tcState.Ccu.Deref.Contents <- ccuContents
diff --git a/src/fsharp/ParseAndCheckInputs.fsi b/src/fsharp/ParseAndCheckInputs.fsi
index d0906edbf27..40ba07d80f7 100644
--- a/src/fsharp/ParseAndCheckInputs.fsi
+++ b/src/fsharp/ParseAndCheckInputs.fsi
@@ -85,6 +85,8 @@ type TcState =
 
     member CreatesGeneratedProvidedTypes: bool
 
+    member RemoveImpl: QualifiedNameOfFile -> TcState
+
 /// Get the initial type checking state for a set of inputs
 val GetInitialTcState: 
     range *
diff --git a/src/fsharp/lib.fsi b/src/fsharp/lib.fsi
index 28d9f10b380..2c30814767a 100644
--- a/src/fsharp/lib.fsi
+++ b/src/fsharp/lib.fsi
@@ -302,6 +302,10 @@ type DisposablesTracker =
 [<RequireQualifiedAccess>]
 module ArrayParallel =
 
+    val inline iter : ('T -> unit) -> 'T [] -> unit
+    
+    val inline iteri : (int -> 'T -> unit) -> 'T [] -> unit
+
     val inline map : ('T -> 'U) -> 'T [] -> 'U []
 
     val inline mapi : (int -> 'T -> 'U) -> 'T [] -> 'U []
diff --git a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj
index 1dd8cacb515..ec7f2fa2f84 100644
--- a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj
+++ b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj
@@ -2,8 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks Condition="'$(OS)' != 'Unix'">net472;net5.0</TargetFrameworks>
-    <TargetFrameworks Condition="'$(OS)' == 'Unix'">net5.0</TargetFrameworks>
+    <TargetFrameworks>net472;net5.0</TargetFrameworks>
     <NoWarn>$(NoWarn);44;75;</NoWarn>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <GenerateProgramFile>false</GenerateProgramFile>
@@ -12,7 +11,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <Compile Include="..\fsharp\TestHelpers.fs" Link="TestHelpers.fs" />
     <Compile Include="LibraryTestFx.fs" />
     <Compile Include="SurfaceArea.netstandard.fs" />
     <Compile Include="..\service\FsUnit.fs">
@@ -72,13 +70,7 @@
     <Compile Include="..\service\ParserTests.fs">
       <Link>ParserTests.fs</Link>
     </Compile>
-    <Compile Include="..\service\XmlDocTests.fs">
-      <Link>XmlDocTests.fs</Link>
-    </Compile>
-    <Compile Include="..\service\PrettyNaming.fs">
-      <Link>PrettyNaming.fs</Link>
-    </Compile>
-    <Compile Include="..\service\Program.fs">
+	<Compile Include="..\service\Program.fs">
       <Link>Program.fs</Link>
     </Compile>
   </ItemGroup>
@@ -86,8 +78,6 @@
   <ItemGroup>
     <PackageReference Include="Dotnet.ProjInfo" Version="0.37.0" />
     <ProjectReference Include="..\..\src\fsharp\FSharp.Compiler.Service\FSharp.Compiler.Service.fsproj" />
-    <ProjectReference Include="..\..\tests\FSharp.Test.Utilities\FSharp.Test.Utilities.fsproj" />
-    <ProjectReference Include="..\service\data\CSharp_Analysis\CSharp_Analysis.csproj" />
   </ItemGroup>
 
-</Project>
+</Project>
\ No newline at end of file
diff --git a/tests/FSharp.Test.Utilities/FSharp.Test.Utilities.fsproj b/tests/FSharp.Test.Utilities/FSharp.Test.Utilities.fsproj
index e180057142e..07250b365ad 100644
--- a/tests/FSharp.Test.Utilities/FSharp.Test.Utilities.fsproj
+++ b/tests/FSharp.Test.Utilities/FSharp.Test.Utilities.fsproj
@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks Condition="'$(OS)' != 'Unix'">net472;net5.0</TargetFrameworks>
+    <TargetFrameworks>net472;net5.0</TargetFrameworks>
     <TargetFrameworks Condition="'$(OS)' == 'Unix'">net5.0</TargetFrameworks>
     <RuntimeIdentifiers>win-x86;win-x64;linux-x64;osx-x64</RuntimeIdentifiers>
     <AssetTargetFallback>$(AssetTargetFallback);portable-net45+win8+wp8+wpa81</AssetTargetFallback>
@@ -28,14 +28,11 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="$(FSharpSourcesRoot)\fsharp\FSharp.Compiler.Service\FSharp.Compiler.Service.fsproj" />
-  </ItemGroup>
-
-  <ItemGroup Condition="'$(SolutionName)' != 'FSharp.Compiler.Service'">
     <!-- CompilerAssert dependencies.
          Make sure they are getting built with the Utilities. -->
     <ProjectReference Include="$(FSharpSourcesRoot)\fsharp\FSharp.Core\FSharp.Core.fsproj" />
     <ProjectReference Include="$(FSharpSourcesRoot)\fsharp\FSharp.Build\FSharp.Build.fsproj" />
+    <ProjectReference Include="$(FSharpSourcesRoot)\fsharp\FSharp.Compiler.Service\FSharp.Compiler.Service.fsproj" />
     <ProjectReference Include="$(FSharpTestsRoot)\fsharpqa\testenv\src\PEVerify\PEVerify.csproj" />
   </ItemGroup>