diff --git a/cli/bench/main.rs b/cli/bench/main.rs
index 06e2d59bb6..8078259d97 100644
--- a/cli/bench/main.rs
+++ b/cli/bench/main.rs
@@ -111,6 +111,11 @@ const EXEC_TIME_BENCHMARKS: &[(&str, &[&str], Option<i32>)] = &[
     &["run", "cli/tests/testdata/text_encoder_into_perf.js"],
     None,
   ),
+  (
+    "response_string",
+    &["run", "cli/tests/testdata/response_string_perf.js"],
+    None,
+  ),
   (
     "check",
     &[
diff --git a/cli/tests/testdata/response_string_perf.js b/cli/tests/testdata/response_string_perf.js
new file mode 100644
index 0000000000..f55376c773
--- /dev/null
+++ b/cli/tests/testdata/response_string_perf.js
@@ -0,0 +1,34 @@
+const mixed = "@Ā๐😀";
+
+function generateRandom(bytes) {
+  let result = "";
+  let i = 0;
+  while (i < bytes) {
+    const toAdd = Math.floor(Math.random() * Math.min(4, bytes - i));
+    switch (toAdd) {
+      case 0:
+        result += mixed[0];
+        i++;
+        break;
+      case 1:
+        result += mixed[1];
+        i++;
+        break;
+      case 2:
+        result += mixed[2];
+        i++;
+        break;
+      case 3:
+        result += mixed[3];
+        result += mixed[4];
+        i += 2;
+        break;
+    }
+  }
+  return result;
+}
+
+const randomData = generateRandom(1024);
+for (let i = 0; i < 10_000; i++) {
+  new Response(randomData);
+}
diff --git a/ext/fetch/22_body.js b/ext/fetch/22_body.js
index 49da149c28..acbef4ec41 100644
--- a/ext/fetch/22_body.js
+++ b/ext/fetch/22_body.js
@@ -372,7 +372,7 @@
     return { body, contentType };
   }
 
-  webidl.converters["BodyInit"] = (V, opts) => {
+  webidl.converters["BodyInit_DOMString"] = (V, opts) => {
     // Union for (ReadableStream or Blob or ArrayBufferView or ArrayBuffer or FormData or URLSearchParams or USVString)
     if (V instanceof ReadableStream) {
       // TODO(lucacasonato): ReadableStream is not branded
@@ -393,10 +393,13 @@
         return webidl.converters["ArrayBufferView"](V, opts);
       }
     }
-    return webidl.converters["USVString"](V, opts);
+    // BodyInit conversion is passed to extractBody(), which calls core.encode().
+    // core.encode() will UTF-8 encode strings with replacement, being equivalent to the USV normalization.
+    // Therefore we can convert to DOMString instead of USVString and avoid a costly redundant conversion.
+    return webidl.converters["DOMString"](V, opts);
   };
-  webidl.converters["BodyInit?"] = webidl.createNullableConverter(
-    webidl.converters["BodyInit"],
+  webidl.converters["BodyInit_DOMString?"] = webidl.createNullableConverter(
+    webidl.converters["BodyInit_DOMString"],
   );
 
   window.__bootstrap.fetchBody = { mixinBody, InnerBody, extractBody };
diff --git a/ext/fetch/23_request.js b/ext/fetch/23_request.js
index 06843d60c7..dfb7e21271 100644
--- a/ext/fetch/23_request.js
+++ b/ext/fetch/23_request.js
@@ -445,7 +445,7 @@
       {
         key: "body",
         converter: webidl.createNullableConverter(
-          webidl.converters["BodyInit"],
+          webidl.converters["BodyInit_DOMString"],
         ),
       },
       { key: "redirect", converter: webidl.converters["RequestRedirect"] },
diff --git a/ext/fetch/23_response.js b/ext/fetch/23_response.js
index 32ce88d424..61e1de151b 100644
--- a/ext/fetch/23_response.js
+++ b/ext/fetch/23_response.js
@@ -253,7 +253,7 @@
      */
     constructor(body = null, init = {}) {
       const prefix = "Failed to construct 'Response'";
-      body = webidl.converters["BodyInit?"](body, {
+      body = webidl.converters["BodyInit_DOMString?"](body, {
         prefix,
         context: "Argument 1",
       });