diff --git a/src/Fable.Cli/CHANGELOG.md b/src/Fable.Cli/CHANGELOG.md
index 3914ef46d..86672406c 100644
--- a/src/Fable.Cli/CHANGELOG.md
+++ b/src/Fable.Cli/CHANGELOG.md
@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * [Rust] Fixed try finally handler order of execution (by @ncave)
 * [JS/TS/Python/Rust] Fixed String.StartsWith/EndsWith (#3934) (by @ncave)
+* [All/Rust] Removed Regex.Replace from hot paths (by @ncave)
 
 ## 4.22.0 - 2024-10-02
 
diff --git a/src/Fable.Transforms/FSharp2Fable.Util.fs b/src/Fable.Transforms/FSharp2Fable.Util.fs
index e5ed029b9..6ed4956ab 100644
--- a/src/Fable.Transforms/FSharp2Fable.Util.fs
+++ b/src/Fable.Transforms/FSharp2Fable.Util.fs
@@ -609,19 +609,27 @@ module Helpers =
             name.Replace('.', '_').Replace('`', '$')
 
     let cleanNameAsRustIdentifier (name: string) =
-        // name |> Naming.sanitizeIdentForbiddenChars
-        let name = Regex.Replace(name, @"[\s`'"".]", "_")
-
         let name =
             if name.Length > 0 && Char.IsDigit(name, 0) then
                 "_" + name
             else
                 name
 
-        let name =
-            Regex.Replace(name, @"[^\w]", (fun c -> String.Format(@"_{0:x4}", int c.Value[0])))
-
-        name
+        if name |> String.exists (fun c -> not (c = '_' || Char.IsLetterOrDigit(c))) then
+            name
+            |> String.collect (
+                function
+                | '_'
+                | ' '
+                | '`'
+                | '.'
+                | '\''
+                | '\"' -> "_"
+                | c when Char.IsLetterOrDigit(c) -> string c
+                | c -> String.Format(@"_{0:x4}", int c)
+            )
+        else
+            name
 
     let memberNameAsRustIdentifier (name: string) part =
         let f = cleanNameAsRustIdentifier
@@ -1692,7 +1700,8 @@ module Identifiers =
             // The F# compiler sometimes adds a numeric suffix. Remove it because it's not deterministic.
             // See https://github.com/fable-compiler/Fable/issues/2869#issuecomment-1169574962
             if fsRef.IsCompilerGenerated then
-                Regex.Replace(fsRef.CompiledName, @"\d+$", "", RegexOptions.Compiled)
+                // Regex.Replace(fsRef.CompiledName, @"\d+$", "", RegexOptions.Compiled)
+                fsRef.CompiledName.TrimEnd([| '0'; '1'; '2'; '3'; '4'; '5'; '6'; '7'; '8'; '9' |])
             else
                 fsRef.CompiledName
 
diff --git a/src/Fable.Transforms/Rust/AST/Rust.AST.Adapters.fs b/src/Fable.Transforms/Rust/AST/Rust.AST.Adapters.fs
index 7940f9bce..c19cd6f8e 100644
--- a/src/Fable.Transforms/Rust/AST/Rust.AST.Adapters.fs
+++ b/src/Fable.Transforms/Rust/AST/Rust.AST.Adapters.fs
@@ -164,34 +164,44 @@ type System.String with
             Some(self.Chars(self.Length - 1))
 
     member self.escape_debug() =
-        // escapes \\, \', \", \t, \r, \n, [\x00-\x1F]
-        let res = self.Replace("\\", @"\\").Replace("\'", @"\'").Replace("\"", @"\""")
-
-        let res = res.Replace("\t", @"\t").Replace("\r", @"\r").Replace("\n", @"\n")
-
-        let res =
-            System.Text.RegularExpressions.Regex.Replace(
-                res,
-                @"[\x00-\x1F]",
-                fun c -> System.String.Format(@"\u{0}{1:x4}{2}", "{", int c.Value[0], "}")
+        if
+            self
+            |> String.exists (fun c -> c = '\\' || c = '\'' || c = '\"' || System.Char.IsControl(c))
+        then
+            self
+            |> String.collect (
+                function
+                | '\t' -> @"\t"
+                | '\r' -> @"\r"
+                | '\n' -> @"\n"
+                | '\\' -> @"\\"
+                | '\'' -> @"\'"
+                | '\"' -> @"\"""
+                | c when System.Char.IsControl(c) -> System.String.Format(@"\u{0}{1:x4}{2}", "{", int c, "}")
+                | c -> string c
             )
-
-        res
+        else
+            self
 
     member self.escape_default() =
-        // escapes \\, \', \", \t, \r, \n, [^\x20-\x7F]
-        let res = self.Replace("\\", @"\\").Replace("\'", @"\'").Replace("\"", @"\""")
-
-        let res = res.Replace("\t", @"\t").Replace("\r", @"\r").Replace("\n", @"\n")
-
-        let res =
-            System.Text.RegularExpressions.Regex.Replace(
-                res,
-                @"[^\x20-\x7F]",
-                fun c -> System.String.Format(@"\u{0}{1:x4}{2}", "{", int c.Value[0], "}")
+        if
+            self
+            |> String.exists (fun c -> c = '\\' || c = '\'' || c = '\"' || c < '\x20' || c > '\x7e')
+        then
+            self
+            |> String.collect (
+                function
+                | '\t' -> @"\t"
+                | '\r' -> @"\r"
+                | '\n' -> @"\n"
+                | '\\' -> @"\\"
+                | '\'' -> @"\'"
+                | '\"' -> @"\"""
+                | c when c < '\x20' || c > '\x7e' -> System.String.Format(@"\u{0}{1:x4}{2}", "{", int c, "}")
+                | c -> string c
             )
-
-        res
+        else
+            self
 
 type System.Text.StringBuilder with
 
diff --git a/src/Fable.Transforms/Rust/Fable2Rust.fs b/src/Fable.Transforms/Rust/Fable2Rust.fs
index 3dd0ffa99..91fe2e366 100644
--- a/src/Fable.Transforms/Rust/Fable2Rust.fs
+++ b/src/Fable.Transforms/Rust/Fable2Rust.fs
@@ -488,8 +488,8 @@ module TypeInfo =
         | _ -> isTypeOfType com isCopyableType isCopyableEntity entNames typ
 
     let isCopyableEntity com entNames (ent: Fable.Entity) =
-        not (ent.IsInterface)
-        && ent.IsValueType
+        ent.IsValueType
+        && not (ent.IsInterface)
         && not (hasMutableFields com ent)
         && (isEntityOfType com isCopyableType entNames ent)
 
@@ -2946,13 +2946,10 @@ module Util =
         let typ = fableExpr.Type
         let nameOpt = tryGetIdentName fableExpr
         let patOpt = makeUnionCasePatOpt com ctx typ nameOpt tag
-
-        match patOpt with
-        | Some pat ->
-            let expr = makeRefForPatternMatch com ctx typ nameOpt fableExpr
-            let letExpr = mkLetExpr pat expr
-            letExpr
-        | _ -> failwith "unreachable"
+        let pat = patOpt |> Option.defaultValue WILD_PAT
+        let expr = makeRefForPatternMatch com ctx typ nameOpt fableExpr
+        let letExpr = mkLetExpr pat expr
+        letExpr
 
     let makeTest isSome thenValue elseValue =
         if isSome then