diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8ac5645a55..830dd37dac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@
   - Fixed tabulation for `throw new Error` bodies
   - Removed empty line at the end of `switch` statement
   - Removed empty `default` case from `switch` statement in the generated code
+- Optimised the Type Extension runtime code and removed trailing `/1` from `RE_EXN_ID` https://github.com/rescript-lang/rescript-compiler/pull/6958
 
 #### :bug: Bug Fix
 - Fix issue where long layout break added a trailing comma in partial application `...`. https://github.com/rescript-lang/rescript-compiler/pull/6949
diff --git a/jscomp/runtime/caml_exceptions.res b/jscomp/runtime/caml_exceptions.res
index 07896a7810..0ca8fee7af 100644
--- a/jscomp/runtime/caml_exceptions.res
+++ b/jscomp/runtime/caml_exceptions.res
@@ -22,42 +22,40 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */
 
-module Map = {
-  type t<'k, 'v>
+type t = {@as("RE_EXN_ID") id: string}
 
-  @new external make: unit => t<'k, 'v> = "Map"
+module Dict = {
+  @obj
+  external empty: unit => dict<'a> = ""
 
-  @send external set: (t<'k, 'v>, 'k, 'v) => unit = "set"
+  @set_index
+  external set: (dict<'a>, string, 'a) => unit = ""
 
-  @send external get: (t<'k, 'v>, 'k) => option<'v> = "get"
+  @get_index
+  /**
+    It's the same as `Js.Dict.get` but it doesn't have runtime overhead to check if the key exists.
+   */
+  external dangerouslyGetNonOption: (dict<'a>, string) => option<'a> = ""
 }
 
-type t = {@as("RE_EXN_ID") id: string}
-
 /**
-   Could be exported for better inlining
-   It's common that we have 
-   {[ a = caml_set_oo_id([248,"string",0]) ]}
-   This can be inlined as 
-   {[ a = caml_set_oo_id([248,"string", caml_oo_last_id++]) ]}
+  Needs to have unique extension ids when used with functors.
+  See discussion in https://github.com/rescript-lang/rescript-compiler/pull/6570
 */
-let idMap: Map.t<string, int> = Map.make()
+let idMap = Dict.empty()
 
 let create = (str: string): string => {
-  let id = switch idMap->Map.get(str) {
+  switch idMap->Dict.dangerouslyGetNonOption(str) {
   | Some(v) => {
       let id = v + 1
-      idMap->Map.set(str, id)
-      id
+      idMap->Dict.set(str, id)
+      str ++ ("/" ++ (Obj.magic((id: int)): string))
     }
   | None => {
-      let id = 1
-      idMap->Map.set(str, id)
-      id
+      idMap->Dict.set(str, 1)
+      str
     }
   }
-
-  str ++ ("/" ++ (Obj.magic((id: int)): string))
 }
 
 /**
diff --git a/lib/es6/caml_exceptions.js b/lib/es6/caml_exceptions.js
index 63b488abac..9db019af3c 100644
--- a/lib/es6/caml_exceptions.js
+++ b/lib/es6/caml_exceptions.js
@@ -1,20 +1,17 @@
 
 
 
-let idMap = new Map();
+let idMap = {};
 
 function create(str) {
-  let v = idMap.get(str);
-  let id;
+  let v = idMap[str];
   if (v !== undefined) {
-    let id$1 = v + 1 | 0;
-    idMap.set(str, id$1);
-    id = id$1;
-  } else {
-    idMap.set(str, 1);
-    id = 1;
+    let id = v + 1 | 0;
+    idMap[str] = id;
+    return str + ("/" + id);
   }
-  return str + ("/" + id);
+  idMap[str] = 1;
+  return str;
 }
 
 function is_extension(e) {
@@ -34,4 +31,4 @@ export {
   is_extension,
   exn_slot_name,
 }
-/* idMap Not a pure module */
+/* No side effect */
diff --git a/lib/es6/caml_js_exceptions.js b/lib/es6/caml_js_exceptions.js
index 878ee0080e..dccd3a3a10 100644
--- a/lib/es6/caml_js_exceptions.js
+++ b/lib/es6/caml_js_exceptions.js
@@ -28,4 +28,4 @@ export {
   internalToOCamlException,
   as_js_exn,
 }
-/* Caml_exceptions Not a pure module */
+/* No side effect */
diff --git a/lib/js/caml_exceptions.js b/lib/js/caml_exceptions.js
index 3150c3c6c0..cdff924b3f 100644
--- a/lib/js/caml_exceptions.js
+++ b/lib/js/caml_exceptions.js
@@ -1,20 +1,17 @@
 'use strict';
 
 
-let idMap = new Map();
+let idMap = {};
 
 function create(str) {
-  let v = idMap.get(str);
-  let id;
+  let v = idMap[str];
   if (v !== undefined) {
-    let id$1 = v + 1 | 0;
-    idMap.set(str, id$1);
-    id = id$1;
-  } else {
-    idMap.set(str, 1);
-    id = 1;
+    let id = v + 1 | 0;
+    idMap[str] = id;
+    return str + ("/" + id);
   }
-  return str + ("/" + id);
+  idMap[str] = 1;
+  return str;
 }
 
 function is_extension(e) {
@@ -32,4 +29,4 @@ function exn_slot_name(x) {
 exports.create = create;
 exports.is_extension = is_extension;
 exports.exn_slot_name = exn_slot_name;
-/* idMap Not a pure module */
+/* No side effect */
diff --git a/lib/js/caml_js_exceptions.js b/lib/js/caml_js_exceptions.js
index 732c63689d..9fe06d0ffa 100644
--- a/lib/js/caml_js_exceptions.js
+++ b/lib/js/caml_js_exceptions.js
@@ -26,4 +26,4 @@ function as_js_exn(exn) {
 exports.$$Error = $$Error;
 exports.internalToOCamlException = internalToOCamlException;
 exports.as_js_exn = as_js_exn;
-/* Caml_exceptions Not a pure module */
+/* No side effect */