diff --git a/eng/Version.Details.xml b/eng/Version.Details.xml
index 231cca712e2..44065c7c818 100644
--- a/eng/Version.Details.xml
+++ b/eng/Version.Details.xml
@@ -3,9 +3,9 @@
   <ProductDependencies>
   </ProductDependencies>
   <ToolsetDependencies>
-    <Dependency Name="Microsoft.DotNet.Arcade.Sdk" Version="1.0.0-beta.20302.3">
+    <Dependency Name="Microsoft.DotNet.Arcade.Sdk" Version="1.0.0-beta.20326.2">
       <Uri>https://github.com/dotnet/arcade</Uri>
-      <Sha>9b71be0663493cd0e111b55536a2e1eeb272f54c</Sha>
+      <Sha>ed69753a3ffbdaa08365252c710d57a64d17f859</Sha>
     </Dependency>
   </ToolsetDependencies>
 </Dependencies>
diff --git a/eng/common/sdl/packages.config b/eng/common/sdl/packages.config
index 256ffbfb93a..968b39bef5f 100644
--- a/eng/common/sdl/packages.config
+++ b/eng/common/sdl/packages.config
@@ -1,4 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
-  <package id="Microsoft.Guardian.Cli" version="0.7.2"/>
+  <package id="Microsoft.Guardian.Cli.win10-x64" version="0.20.1"/>
 </packages>
diff --git a/eng/common/templates/job/execute-sdl.yml b/eng/common/templates/job/execute-sdl.yml
index 52e2ff021d7..bf09d2511c6 100644
--- a/eng/common/templates/job/execute-sdl.yml
+++ b/eng/common/templates/job/execute-sdl.yml
@@ -65,7 +65,7 @@ jobs:
       continueOnError: ${{ parameters.sdlContinueOnError }}
   - ${{ if eq(parameters.overrideParameters, '') }}:
     - powershell: eng/common/sdl/execute-all-sdl-tools.ps1
-        -GuardianPackageName Microsoft.Guardian.Cli.0.7.2
+        -GuardianPackageName Microsoft.Guardian.Cli.win10-x64.0.20.1
         -NugetPackageDirectory $(Build.SourcesDirectory)\.packages
         -AzureDevOpsAccessToken $(dn-bot-dotnet-build-rw-code-rw)
         ${{ parameters.additionalParameters }}
diff --git a/global.json b/global.json
index 146979f9b93..7afbdb52d28 100644
--- a/global.json
+++ b/global.json
@@ -9,7 +9,7 @@
     }
   },
   "msbuild-sdks": {
-    "Microsoft.DotNet.Arcade.Sdk": "1.0.0-beta.20302.3",
+    "Microsoft.DotNet.Arcade.Sdk": "1.0.0-beta.20326.2",
     "Microsoft.DotNet.Helix.Sdk": "2.0.0-beta.19069.2"
   }
 }
diff --git a/src/fsharp/FSharp.Core/string.fs b/src/fsharp/FSharp.Core/string.fs
index 7e4b65f1a08..d009615e6aa 100644
--- a/src/fsharp/FSharp.Core/string.fs
+++ b/src/fsharp/FSharp.Core/string.fs
@@ -12,6 +12,11 @@ namespace Microsoft.FSharp.Core
     [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
     [<RequireQualifiedAccess>]
     module String =
+        [<Literal>]
+        /// LOH threshold is calculated from FSharp.Compiler.AbstractIL.Internal.Library.LOH_SIZE_THRESHOLD_BYTES,
+        /// and is equal to 80_000 / sizeof<char>
+        let LOH_CHAR_THRESHOLD = 40_000
+
         [<CompiledName("Length")>]
         let length (str:string) = if isNull str then 0 else str.Length
 
@@ -37,9 +42,13 @@ namespace Microsoft.FSharp.Core
             if String.IsNullOrEmpty str then
                 String.Empty
             else
-                let res = StringBuilder str.Length
-                str |> iter (fun c -> res.Append(mapping c) |> ignore)
-                res.ToString()
+                let result = str.ToCharArray()
+                let mutable i = 0
+                for c in result do
+                    result.[i] <- mapping c
+                    i <- i + 1
+
+                new String(result)
 
         [<CompiledName("MapIndexed")>]
         let mapi (mapping: int -> char -> char) (str:string) =
@@ -53,13 +62,30 @@ namespace Microsoft.FSharp.Core
 
         [<CompiledName("Filter")>]
         let filter (predicate: char -> bool) (str:string) =
-            if String.IsNullOrEmpty str then
+            let len = length str
+
+            if len = 0 then 
                 String.Empty
-            else
-                let res = StringBuilder str.Length
+
+            elif len > LOH_CHAR_THRESHOLD then
+                // By using SB here, which is twice slower than the optimized path, we prevent LOH allocations 
+                // and 'stop the world' collections if the filtering results in smaller strings.
+                // We also don't pre-allocate SB here, to allow for less mem pressure when filter result is small.
+                let res = StringBuilder()
                 str |> iter (fun c -> if predicate c then res.Append c |> ignore)
                 res.ToString()
 
+            else
+                // Must do it this way, since array.fs is not yet in scope, but this is safe
+                let target = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked len
+                let mutable i = 0
+                for c in str do
+                    if predicate c then 
+                        target.[i] <- c
+                        i <- i + 1
+
+                String(target, 0, i)
+
         [<CompiledName("Collect")>]
         let collect (mapping: char -> string) (str:string) =
             if String.IsNullOrEmpty str then
@@ -81,13 +107,38 @@ namespace Microsoft.FSharp.Core
         let replicate (count:int) (str:string) =
             if count < 0 then invalidArgInputMustBeNonNegative "count" count
 
-            if String.IsNullOrEmpty str then
+            let len = length str
+            if len = 0 || count = 0 then 
                 String.Empty
+
+            elif len = 1 then
+                new String(str.[0], count)
+
+            elif count <= 4 then
+                match count with
+                | 1 -> str
+                | 2 -> String.Concat(str, str)
+                | 3 -> String.Concat(str, str, str)
+                | _ -> String.Concat(str, str, str, str)
+
             else
-                let res = StringBuilder(count * str.Length)
-                for i = 0 to count - 1 do 
-                   res.Append str |> ignore
-                res.ToString()
+                // Using the primitive, because array.fs is not yet in scope. It's safe: both len and count are positive.
+                let target = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked (len * count)
+                let source = str.ToCharArray()
+
+                // O(log(n)) performance loop:
+                // Copy first string, then keep copying what we already copied 
+                // (i.e., doubling it) until we reach or pass the halfway point
+                Array.Copy(source, 0, target, 0, len)
+                let mutable i = len
+                while i * 2 < target.Length do
+                    Array.Copy(target, 0, target, i, i)
+                    i <- i * 2
+
+                // finally, copy the remain half, or less-then half
+                Array.Copy(target, 0, target, i, target.Length - i)
+                new String(target)
+
 
         [<CompiledName("ForAll")>]
         let forall predicate (str:string) =
diff --git a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs
index a72ab64e236..6c1b1360585 100644
--- a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs
+++ b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs
@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
 
 namespace FSharp.Core.UnitTests.Collections
 
@@ -71,11 +71,35 @@ type StringModule() =
 
     [<Test>]
     member this.Map() =
-        let e1 = String.map (fun c -> c) "foo"
-        Assert.AreEqual("foo", e1)
+        let e1 = String.map id "xyz"
+        Assert.AreEqual("xyz", e1)
 
-        let e2 = String.map (fun c -> c) null 
-        Assert.AreEqual("", e2)
+        let e2 = String.map (fun c -> c + char 1) "abcde"
+        Assert.AreEqual("bcdef", e2)
+
+        let e3 = String.map (fun c -> c) null 
+        Assert.AreEqual("", e3)
+
+        let e4 = String.map (fun c -> c) String.Empty 
+        Assert.AreEqual("", e4)
+
+        let e5 = String.map (fun _ -> 'B') "A"
+        Assert.AreEqual("B", e5)
+
+        let e6 = String.map (fun _ -> failwith "should not raise") null
+        Assert.AreEqual("", e6)
+
+        // this tests makes sure mapping function is not called too many times
+        let mutable x = 0
+        let e7 = String.map (fun _ -> if x > 2 then failwith "should not raise" else x <- x + 1; 'x') "abc"
+        Assert.AreEqual(x, 3)
+        Assert.AreEqual(e7, "xxx")
+
+        // side-effect and "order of operation" test
+        let mutable x = 0
+        let e8 = String.map (fun c -> x <- x + 1; c + char x) "abcde"
+        Assert.AreEqual(x, 5)
+        Assert.AreEqual(e8, "bdfhj")
 
     [<Test>]
     member this.MapI() =
@@ -87,18 +111,25 @@ type StringModule() =
 
     [<Test>]
     member this.Filter() =
-        let e1 = String.filter (fun c -> true) "foo"
-        Assert.AreEqual("foo", e1)
+        let e1 = String.filter (fun c -> true) "Taradiddle"
+        Assert.AreEqual("Taradiddle", e1)
 
         let e2 = String.filter (fun c -> true) null 
         Assert.AreEqual("", e2)
 
-        let e3 = String.filter (fun c -> c <> 'o') "foo bar"
-        Assert.AreEqual("f bar", e3)
+        let e3 = String.filter Char.IsUpper "How Vexingly Quick Daft Zebras Jump!"
+        Assert.AreEqual("HVQDZJ", e3)
 
         let e4 = String.filter (fun c -> c <> 'o') ""
         Assert.AreEqual("", e4)
 
+        let e5 = String.filter (fun c -> c > 'B' ) "ABRACADABRA"
+        Assert.AreEqual("RCDR", e5)
+
+        // LOH test with 55k string, which is 110k bytes
+        let e5 = String.filter (fun c -> c > 'B' ) (String.replicate 5_000 "ABRACADABRA")
+        Assert.AreEqual(String.replicate 5_000 "RCDR", e5)
+
     [<Test>]
     member this.Collect() =
         let e1 = String.collect (fun c -> "a"+string c) "foo"
@@ -125,15 +156,42 @@ type StringModule() =
 
     [<Test>]
     member this.Replicate() = 
-        let e1 = String.replicate 0 "foo"
+        let e1 = String.replicate 0 "Snickersnee"
         Assert.AreEqual("", e1)
 
-        let e2 = String.replicate 2 "foo"
-        Assert.AreEqual("foofoo", e2)
+        let e2 = String.replicate 2 "Collywobbles, "
+        Assert.AreEqual("Collywobbles, Collywobbles, ", e2)
 
         let e3 = String.replicate 2 null
         Assert.AreEqual("", e3)
 
+        let e4 = String.replicate 300_000 ""
+        Assert.AreEqual("", e4)
+
+        let e5 = String.replicate 23 "天地玄黃，宇宙洪荒。"
+        Assert.AreEqual(230 , e5.Length)
+        Assert.AreEqual("天地玄黃，宇宙洪荒。天地玄黃，宇宙洪荒。", e5.Substring(0, 20))
+
+        // This tests the cut-off point for the O(log(n)) algorithm with a prime number
+        let e6 = String.replicate 84673 "!!!"
+        Assert.AreEqual(84673 * 3, e6.Length)
+
+        // This tests the cut-off point for the O(log(n)) algorithm with a 2^x number
+        let e7 = String.replicate 1024 "!!!"
+        Assert.AreEqual(1024 * 3, e7.Length)
+
+        let e8 = String.replicate 1 "What a wonderful world"
+        Assert.AreEqual("What a wonderful world", e8)
+
+        let e9 = String.replicate 3 "أضعت طريقي! أضعت طريقي"  // means: I'm lost
+        Assert.AreEqual("أضعت طريقي! أضعت طريقيأضعت طريقي! أضعت طريقيأضعت طريقي! أضعت طريقي", e9)
+
+        let e10 = String.replicate 4 "㏖ ㏗ ℵ "
+        Assert.AreEqual("㏖ ㏗ ℵ ㏖ ㏗ ℵ ㏖ ㏗ ℵ ㏖ ㏗ ℵ ", e10)
+
+        let e11 = String.replicate 5 "5"
+        Assert.AreEqual("55555", e11)
+
         CheckThrowsArgumentException(fun () -> String.replicate -1 "foo" |> ignore)
 
     [<Test>]