Improve performance of String.concat for arrays and lists

Author: abelbraaksma

src/fsharp/FSharp.Core/string.fs

@@ -8,16 +8,34 @@ namespace Microsoft.FSharp.Core
     open Microsoft.FSharp.Core.Operators
     open Microsoft.FSharp.Core.Operators.Checked
     open Microsoft.FSharp.Collections
+    open Microsoft.FSharp.Primitives.Basics
 
     [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
     [<RequireQualifiedAccess>]
     module String =
         [<CompiledName("Length")>]
         let length (str:string) = if isNull str then 0 else str.Length
 
+        let private concatArray sep (strings: string []) =
+            match length sep with
+            | 0 -> String.Concat strings
+            // following line should be used when this overload becomes part of .NET Standard (it's only in .NET Core)
+            //| 1 -> String.Join(sep.[0], strings, 0, strings.Length)
+            | _ -> String.Join(sep, strings, 0, strings.Length)
+
         [<CompiledName("Concat")>]
         let concat sep (strings : seq<string>) =  
-            String.Join(sep, strings)
+            match strings with
+            | :? array<string> as arr -> 
+                concatArray sep arr
+
+            | :? list<string> as lst -> 
+                lst 
+                |> List.toArray 
+                |> concatArray sep
+
+            | _ ->
+                String.Join(sep, strings)
 
         [<CompiledName("Iterate")>]
         let iter (action : (char -> unit)) (str:string) =