diff --git a/src/Compiler/Driver/CompilerConfig.fs b/src/Compiler/Driver/CompilerConfig.fs
index 08946b84f63..91a2e9fde3d 100644
--- a/src/Compiler/Driver/CompilerConfig.fs
+++ b/src/Compiler/Driver/CompilerConfig.fs
@@ -1405,14 +1405,13 @@ type TcConfig private (data: TcConfigBuilder, validate: bool) =
     /// 'framework' reference set that is potentially shared across multiple compilations.
     member tcConfig.IsSystemAssembly(fileName: string) =
         try
+            let dirName = Path.GetDirectoryName fileName
+            let baseName = FileSystemUtils.fileNameWithoutExtension fileName
+
             FileSystem.FileExistsShim fileName
-            && ((tcConfig.GetTargetFrameworkDirectories()
-                 |> List.exists (fun clrRoot -> clrRoot = Path.GetDirectoryName fileName))
-                || (tcConfig
-                       .FxResolver
-                       .GetSystemAssemblies()
-                       .Contains(FileSystemUtils.fileNameWithoutExtension fileName))
-                || tcConfig.FxResolver.IsInReferenceAssemblyPackDirectory fileName)
+            && ((tcConfig.GetTargetFrameworkDirectories() |> List.contains dirName)
+                || FxResolver.GetSystemAssemblies().Contains baseName
+                || FxResolver.IsReferenceAssemblyPackDirectoryApprox dirName)
         with _ ->
             false
 
diff --git a/src/Compiler/Driver/FxResolver.fs b/src/Compiler/Driver/FxResolver.fs
index 2fa595ee15b..ce2474a7085 100644
--- a/src/Compiler/Driver/FxResolver.fs
+++ b/src/Compiler/Driver/FxResolver.fs
@@ -611,7 +611,7 @@ type internal FxResolver
     // A set of assemblies to always consider to be system assemblies.  A common set of these can be used a shared
     // resources between projects in the compiler services.  Also all assemblies where well-known system types exist
     // referenced from TcGlobals must be listed here.
-    let systemAssemblies =
+    static let systemAssemblies =
         HashSet
             [
                 // NOTE: duplicates are ok in this list
@@ -789,17 +789,10 @@ type internal FxResolver
                 "WindowsBase"
             ]
 
-    member _.GetSystemAssemblies() = systemAssemblies
+    static member GetSystemAssemblies() = systemAssemblies
 
-    member _.IsInReferenceAssemblyPackDirectory fileName =
-        fxlock.AcquireLock(fun fxtok ->
-            RequireFxResolverLock(fxtok, "assuming all member require lock")
-
-            match tryGetNetCoreRefsPackDirectoryRoot () |> replayWarnings with
-            | _, Some root ->
-                let path = Path.GetDirectoryName(fileName)
-                path.StartsWith(root, StringComparison.OrdinalIgnoreCase)
-            | _ -> false)
+    static member IsReferenceAssemblyPackDirectoryApprox(dirName: string) =
+        dirName.Contains "Microsoft.NETCore.App.Ref"
 
     member _.TryGetSdkDir() =
         fxlock.AcquireLock(fun fxtok ->
diff --git a/src/Compiler/Driver/FxResolver.fsi b/src/Compiler/Driver/FxResolver.fsi
index 2bca0a75d8c..d740d2fc497 100644
--- a/src/Compiler/Driver/FxResolver.fsi
+++ b/src/Compiler/Driver/FxResolver.fsi
@@ -28,12 +28,14 @@ type internal FxResolver =
 
     member GetFrameworkRefsPackDirectory: unit -> string option
 
-    member GetSystemAssemblies: unit -> HashSet<string>
+    static member GetSystemAssemblies: unit -> HashSet<string>
 
     /// Gets the selected target framework moniker, e.g netcore3.0, net472, and the running rid of the current machine
     member GetTfmAndRid: unit -> string * string
 
-    member IsInReferenceAssemblyPackDirectory: fileName: string -> bool
+    /// Determines if an assembly is in the core set of assemblies with high likelihood of
+    /// being shared amongst a set of common scripting references
+    static member IsReferenceAssemblyPackDirectoryApprox: dirName: string -> bool
 
     member TryGetDesiredDotNetSdkVersionForDirectory: unit -> Result<string, exn>