diff --git a/benchmarks/fetch/isomorphic-encode.mjs b/benchmarks/fetch/isomorphic-encode.mjs
new file mode 100644
index 00000000000..bf772390bba
--- /dev/null
+++ b/benchmarks/fetch/isomorphic-encode.mjs
@@ -0,0 +1,75 @@
+import { bench, group, run } from 'mitata'
+import { isomorphicEncode } from '../../lib/web/fetch/util.js'
+
+const characters =
+  'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'
+const charactersLength = characters.length
+
+function generateAsciiString (length) {
+  let result = ''
+  for (let i = 0; i < length; ++i) {
+    result += characters[Math.floor(Math.random() * charactersLength)]
+  }
+  return result
+}
+
+const invalidIsomorphicEncodeValueRegex = /[^\x00-\xFF]/ // eslint-disable-line
+
+function isomorphicEncode1 (input) {
+  for (let i = 0; i < input.length; i++) {
+    if (input.charCodeAt(i) > 0xff) {
+      throw new TypeError('Unreachable')
+    }
+  }
+  return input
+}
+
+function isomorphicEncode2 (input) {
+  if (invalidIsomorphicEncodeValueRegex.test(input)) {
+    throw new TypeError('Unreachable')
+  }
+  return input
+}
+
+const settings = {
+  small: 10,
+  middle: 30,
+  long: 70
+}
+
+for (const [runName, length] of Object.entries(settings)) {
+  const value = generateAsciiString(length);
+
+  [
+    { name: `${runName} (valid)`, value },
+    {
+      name: `${runName} (invalid)`,
+      value: `${value.slice(0, -1)}${String.fromCharCode(0xff + 1)}`
+    }
+  ].forEach(({ name, value }) => {
+    group(name, () => {
+      [
+        {
+          name: 'original',
+          fn: isomorphicEncode
+        },
+        {
+          name: 'String#charCodeAt',
+          fn: isomorphicEncode1
+        },
+        {
+          name: 'RegExp#test',
+          fn: isomorphicEncode2
+        }
+      ].forEach(({ name, fn }) => {
+        bench(name, () => {
+          try {
+            return fn(value)
+          } catch (err) {}
+        })
+      })
+    })
+  })
+}
+
+await run()
diff --git a/lib/web/fetch/util.js b/lib/web/fetch/util.js
index 7dd56c1d5c1..8c73f909604 100644
--- a/lib/web/fetch/util.js
+++ b/lib/web/fetch/util.js
@@ -1098,15 +1098,15 @@ function readableStreamClose (controller) {
   }
 }
 
+const invalidIsomorphicEncodeValueRegex = /[^\x00-\xFF]/ // eslint-disable-line
+
 /**
  * @see https://infra.spec.whatwg.org/#isomorphic-encode
  * @param {string} input
  */
 function isomorphicEncode (input) {
   // 1. Assert: input contains no code points greater than U+00FF.
-  for (let i = 0; i < input.length; i++) {
-    assert(input.charCodeAt(i) <= 0xFF)
-  }
+  assert(!invalidIsomorphicEncodeValueRegex.test(input))
 
   // 2. Return a byte sequence whose length is equal to input’s code
   //    point length and whose bytes have the same values as the