diff --git a/lib/internal/url.js b/lib/internal/url.js
index e96df6148f98..dcd48d15a7a4 100644
--- a/lib/internal/url.js
+++ b/lib/internal/url.js
@@ -74,14 +74,10 @@ const {
   },
 } = require('internal/errors');
 const {
-  CHAR_AMPERSAND,
   CHAR_BACKWARD_SLASH,
-  CHAR_EQUAL,
   CHAR_FORWARD_SLASH,
   CHAR_LOWERCASE_A,
   CHAR_LOWERCASE_Z,
-  CHAR_PERCENT,
-  CHAR_PLUS,
   CHAR_COLON,
 } = require('internal/constants');
 const path = require('path');
@@ -331,12 +327,17 @@ class URLSearchParams {
   // "associated url object"
   #context;
 
+  // Cached application/x-www-form-urlencoded serialization. Cleared on
+  // mutation so repeated toString()/URL.href reads stay cheap.
+  #serialized;
+
   static {
     setURLSearchParamsContext = (obj, ctx) => {
       obj.#context = ctx;
     };
     getURLSearchParamsList = (obj) => obj.#searchParams;
     setURLSearchParams = (obj, query) => {
+      obj.#serialized = undefined;
       if (query === undefined) {
         obj.#searchParams = [];
       } else {
@@ -345,6 +346,13 @@ class URLSearchParams {
     };
   }
 
+  #markUpdated() {
+    this.#serialized = undefined;
+    if (this.#context) {
+      setURLSearchParamsModified(this.#context);
+    }
+  }
+
   // URL Standard says the default value is '', but as undefined and '' have
   // the same result, undefined is used to prevent unnecessary parsing.
   // Default parameter is necessary to keep URLSearchParams.length === 0 in
@@ -361,6 +369,7 @@ class URLSearchParams {
         // shortcut to avoid having to go through the costly generic iterator.
         const childParams = init.#searchParams;
         this.#searchParams = childParams.slice();
+        this.#serialized = init.#serialized;
       } else if (method != null) {
         // Sequence<sequence<USVString>>
         if (typeof method !== 'function') {
@@ -388,8 +397,8 @@ class URLSearchParams {
             // Append (innerSequence[0], innerSequence[1]) to query's list.
             ArrayPrototypePush(
               this.#searchParams,
-              StringPrototypeToWellFormed(`${pair[0]}`),
-              StringPrototypeToWellFormed(`${pair[1]}`),
+              toUSVString(pair[0]),
+              toUSVString(pair[1]),
             );
           } else {
             if (((typeof pair !== 'object' && typeof pair !== 'function') ||
@@ -401,7 +410,7 @@ class URLSearchParams {
 
             for (const element of pair) {
               length++;
-              ArrayPrototypePush(this.#searchParams, StringPrototypeToWellFormed(`${element}`));
+              ArrayPrototypePush(this.#searchParams, toUSVString(element));
             }
 
             // If innerSequence's size is not 2, then throw a TypeError.
@@ -419,8 +428,8 @@ class URLSearchParams {
           const key = keys[i];
           const desc = ReflectGetOwnPropertyDescriptor(init, key);
           if (desc !== undefined && desc.enumerable) {
-            const typedKey = StringPrototypeToWellFormed(key);
-            const typedValue = StringPrototypeToWellFormed(`${init[key]}`);
+            const typedKey = toUSVString(key);
+            const typedValue = toUSVString(init[key]);
 
             // Two different keys may become the same USVString after normalization.
             // In that case, we retain the later one. Refer to WPT.
@@ -437,7 +446,7 @@ class URLSearchParams {
       }
     } else {
       // https://url.spec.whatwg.org/#dom-urlsearchparams-urlsearchparams
-      init = StringPrototypeToWellFormed(`${init}`);
+      init = toUSVString(init);
       this.#searchParams = init ? parseParams(init) : [];
     }
   }
@@ -491,13 +500,10 @@ class URLSearchParams {
       throw new ERR_MISSING_ARGS('name', 'value');
     }
 
-    name = StringPrototypeToWellFormed(`${name}`);
-    value = StringPrototypeToWellFormed(`${value}`);
+    name = toUSVString(name);
+    value = toUSVString(value);
     ArrayPrototypePush(this.#searchParams, name, value);
-
-    if (this.#context) {
-      setURLSearchParamsModified(this.#context);
-    }
+    this.#markUpdated();
   }
 
   delete(name, value = undefined) {
@@ -509,12 +515,12 @@ class URLSearchParams {
     }
 
     const list = this.#searchParams;
-    name = StringPrototypeToWellFormed(`${name}`);
+    name = toUSVString(name);
     const { length } = list;
     let write = 0;
 
     if (value !== undefined) {
-      value = StringPrototypeToWellFormed(`${value}`);
+      value = toUSVString(value);
       for (let i = 0; i < length; i += 2) {
         if (list[i] === name && list[i + 1] === value) {
           continue;
@@ -538,8 +544,10 @@ class URLSearchParams {
       }
     }
 
-    if (write !== length)
+    if (write !== length) {
       list.length = write;
+      this.#serialized = undefined;
+    }
 
     if (this.#context) {
       setURLSearchParamsModified(this.#context);
@@ -555,8 +563,9 @@ class URLSearchParams {
     }
 
     const list = this.#searchParams;
-    name = StringPrototypeToWellFormed(`${name}`);
-    for (let i = 0; i < list.length; i += 2) {
+    name = toUSVString(name);
+    const { length } = list;
+    for (let i = 0; i < length; i += 2) {
       if (list[i] === name) {
         return list[i + 1];
       }
@@ -574,10 +583,11 @@ class URLSearchParams {
 
     const list = this.#searchParams;
     const values = [];
-    name = StringPrototypeToWellFormed(`${name}`);
-    for (let i = 0; i < list.length; i += 2) {
+    name = toUSVString(name);
+    const { length } = list;
+    for (let i = 0; i < length; i += 2) {
       if (list[i] === name) {
-        values.push(list[i + 1]);
+        ArrayPrototypePush(values, list[i + 1]);
       }
     }
     return values;
@@ -592,13 +602,14 @@ class URLSearchParams {
     }
 
     const list = this.#searchParams;
-    name = StringPrototypeToWellFormed(`${name}`);
+    name = toUSVString(name);
 
     if (value !== undefined) {
-      value = StringPrototypeToWellFormed(`${value}`);
+      value = toUSVString(value);
     }
 
-    for (let i = 0; i < list.length; i += 2) {
+    const { length } = list;
+    for (let i = 0; i < length; i += 2) {
       if (list[i] === name) {
         if (value === undefined || list[i + 1] === value) {
           return true;
@@ -618,8 +629,8 @@ class URLSearchParams {
     }
 
     const list = this.#searchParams;
-    name = StringPrototypeToWellFormed(`${name}`);
-    value = StringPrototypeToWellFormed(`${value}`);
+    name = toUSVString(name);
+    value = toUSVString(value);
     const { length } = list;
 
     // If there are any name-value pairs whose name is `name`, in `list`, set
@@ -656,9 +667,7 @@ class URLSearchParams {
       ArrayPrototypePush(list, name, value);
     }
 
-    if (this.#context) {
-      setURLSearchParamsModified(this.#context);
-    }
+    this.#markUpdated();
   }
 
   sort() {
@@ -705,9 +714,7 @@ class URLSearchParams {
       }
     }
 
-    if (this.#context) {
-      setURLSearchParamsModified(this.#context);
-    }
+    this.#markUpdated();
   }
 
   // https://heycam.github.io/webidl/#es-iterators
@@ -760,7 +767,12 @@ class URLSearchParams {
     if (typeof this !== 'object' || this === null || !(#searchParams in this))
       throw new ERR_INVALID_THIS('URLSearchParams');
 
-    return serializeParams(this.#searchParams);
+    if (this.#serialized !== undefined) {
+      return this.#serialized;
+    }
+    const serialized = serializeParams(this.#searchParams);
+    this.#serialized = serialized;
+    return serialized;
   }
 }
 
@@ -1276,102 +1288,82 @@ function installObjectURLMethods() {
   });
 }
 
+function toUSVString(value) {
+  return typeof value === 'string' ?
+    StringPrototypeToWellFormed(value) :
+    StringPrototypeToWellFormed(`${value}`);
+}
+
+function unescapeFormComponent(s) {
+  try {
+    return decodeURIComponent(s);
+  } catch {
+    return querystring.unescapeBuffer(s).toString();
+  }
+}
+
+function hasPercentHex(s) {
+  const end = s.length - 2;
+  for (let i = 0; i < end; i++) {
+    if (StringPrototypeCharCodeAt(s, i) === 37 && // '%'
+        isHexTable[StringPrototypeCharCodeAt(s, i + 1)] === 1 &&
+        isHexTable[StringPrototypeCharCodeAt(s, i + 2)] === 1) {
+      return true;
+    }
+  }
+  return false;
+}
+
+function decodeFormComponent(qs, start, end) {
+  if (start >= end) {
+    return '';
+  }
+  const s = qs.slice(start, end);
+  const plus = s.indexOf('+');
+  const pct = s.indexOf('%');
+  if (plus === -1 && pct === -1) {
+    return s;
+  }
+  const replaced = plus === -1 ? s : s.replaceAll('+', ' ');
+  // Only percent-decode when a complete %HH sequence exists. A lone '%' or
+  // a '%' followed by a non-hex character must be left intact so later
+  // serialization can encode the raw bytes.
+  if (pct === -1 || !hasPercentHex(replaced)) {
+    return replaced;
+  }
+  return unescapeFormComponent(replaced);
+}
+
 // application/x-www-form-urlencoded parser
 // Ref: https://url.spec.whatwg.org/#concept-urlencoded-parser
 function parseParams(qs) {
-  const out = [];
-  let seenSep = false;
-  let buf = '';
-  let encoded = false;
-  let encodeCheck = 0;
+  const len = qs.length;
   let i = qs[0] === '?' ? 1 : 0;
-  let pairStart = i;
-  let lastPos = i;
-  for (; i < qs.length; ++i) {
-    const code = StringPrototypeCharCodeAt(qs, i);
-
-    // Try matching key/value pair separator
-    if (code === CHAR_AMPERSAND) {
-      if (pairStart === i) {
-        // We saw an empty substring between pair separators
-        lastPos = pairStart = i + 1;
-        continue;
-      }
+  if (i >= len) {
+    return [];
+  }
 
-      if (lastPos < i)
-        buf += qs.slice(lastPos, i);
-      if (encoded)
-        buf = querystring.unescape(buf);
-      out.push(buf);
-
-      // If `buf` is the key, add an empty value.
-      if (!seenSep)
-        out.push('');
-
-      seenSep = false;
-      buf = '';
-      encoded = false;
-      encodeCheck = 0;
-      lastPos = pairStart = i + 1;
-      continue;
-    }
-
-    // Try matching key/value separator (e.g. '=') if we haven't already
-    if (!seenSep && code === CHAR_EQUAL) {
-      // Key/value separator match!
-      if (lastPos < i)
-        buf += qs.slice(lastPos, i);
-      if (encoded)
-        buf = querystring.unescape(buf);
-      out.push(buf);
-
-      seenSep = true;
-      buf = '';
-      encoded = false;
-      encodeCheck = 0;
-      lastPos = i + 1;
-      continue;
-    }
-
-    // Handle + and percent decoding.
-    if (code === CHAR_PLUS) {
-      if (lastPos < i)
-        buf += StringPrototypeSlice(qs, lastPos, i);
-      buf += ' ';
-      lastPos = i + 1;
-    } else if (!encoded) {
-      // Try to match an (valid) encoded byte (once) to minimize unnecessary
-      // calls to string decoding functions
-      if (code === CHAR_PERCENT) {
-        encodeCheck = 1;
-      } else if (encodeCheck > 0) {
-        if (isHexTable[code] === 1) {
-          if (++encodeCheck === 3) {
-            encoded = true;
-          }
-        } else {
-          encodeCheck = 0;
-        }
+  const out = [];
+  // Native indexOf/slice/push outperform primordials on this tight loop.
+  const encoded = qs.indexOf('+', i) !== -1 || qs.indexOf('%', i) !== -1;
+  while (i < len) {
+    let amp = qs.indexOf('&', i);
+    if (amp === -1) {
+      amp = len;
+    }
+    if (amp !== i) {
+      const eq = qs.indexOf('=', i);
+      if (eq === -1 || eq > amp) {
+        out.push(encoded ? decodeFormComponent(qs, i, amp) : qs.slice(i, amp), '');
+      } else {
+        out.push(
+          encoded ? decodeFormComponent(qs, i, eq) : qs.slice(i, eq),
+          encoded ? decodeFormComponent(qs, eq + 1, amp) : qs.slice(eq + 1, amp),
+        );
       }
     }
+    i = amp + 1;
   }
-
-  // Deal with any leftover key or value data
-
-  // There is a trailing &. No more processing is needed.
-  if (pairStart === i)
-    return out;
-
-  if (lastPos < i)
-    buf += StringPrototypeSlice(qs, lastPos, i);
-  if (encoded)
-    buf = querystring.unescape(buf);
-  ArrayPrototypePush(out, buf);
-
-  // If `buf` is the key, add an empty value.
-  if (!seenSep)
-    ArrayPrototypePush(out, '');
-
   return out;
 }
 
@@ -1402,17 +1394,17 @@ function serializeParams(array) {
   if (len === 0)
     return '';
 
-  const firstEncodedParam = encodeStr(array[0], noEscape, paramHexTable);
-  const firstEncodedValue = encodeStr(array[1], noEscape, paramHexTable);
-  let output = `${firstEncodedParam}=${firstEncodedValue}`;
-
-  for (let i = 2; i < len; i += 2) {
-    const encodedParam = encodeStr(array[i], noEscape, paramHexTable);
-    const encodedValue = encodeStr(array[i + 1], noEscape, paramHexTable);
-    output += `&${encodedParam}=${encodedValue}`;
+  if (len === 2) {
+    return encodeStr(array[0], noEscape, paramHexTable) + '=' +
+           encodeStr(array[1], noEscape, paramHexTable);
   }
 
-  return output;
+  const pairs = new Array(len / 2);
+  for (let i = 0, j = 0; i < len; i += 2, ++j) {
+    pairs[j] = encodeStr(array[i], noEscape, paramHexTable) + '=' +
+               encodeStr(array[i + 1], noEscape, paramHexTable);
+  }
+  return ArrayPrototypeJoin(pairs, '&');
 }
 
 // for merge sort
diff --git a/test/parallel/test-whatwg-url-searchparams-fast-path.js b/test/parallel/test-whatwg-url-searchparams-fast-path.js
new file mode 100644
index 000000000000..2dbb38f88aa5
--- /dev/null
+++ b/test/parallel/test-whatwg-url-searchparams-fast-path.js
@@ -0,0 +1,116 @@
+'use strict';
+
+// Tests for the URLSearchParams parse / serialize / toUSVString fast paths.
+
+require('../common');
+const assert = require('assert');
+
+{
+  const params = new URLSearchParams('?a=b');
+  assert.strictEqual(params.toString(), 'a=b');
+  assert.strictEqual(params.get('a'), 'b');
+}
+
+{
+  const params = new URLSearchParams('a=b&c');
+  assert.deepStrictEqual([...params], [['a', 'b'], ['c', '']]);
+}
+
+{
+  const params = new URLSearchParams('&a&&& &&&&&a+b=& c&m%c3%b8%c3%b8');
+  assert.ok(params.has('a'));
+  assert.ok(params.has('a b'));
+  assert.ok(params.has(' '));
+  assert.ok(params.has(' c'));
+  assert.ok(params.has('møø'));
+  assert.strictEqual(params.get('a+b'), null);
+}
+
+{
+  const params = new URLSearchParams('id=0&value=%');
+  assert.strictEqual(params.get('id'), '0');
+  assert.strictEqual(params.get('value'), '%');
+}
+
+{
+  const params = new URLSearchParams('b=%2sf%2a');
+  assert.strictEqual(params.get('b'), '%2sf*');
+}
+
+{
+  const params = new URLSearchParams('a=b=c&d=');
+  assert.strictEqual(params.get('a'), 'b=c');
+  assert.strictEqual(params.get('d'), '');
+}
+
+{
+  const params = new URLSearchParams('foo=bar&baz=quux');
+  assert.strictEqual(params.toString(), 'foo=bar&baz=quux');
+  assert.strictEqual(params.toString(), 'foo=bar&baz=quux');
+  params.append('xyzzy', 'thud');
+  assert.strictEqual(params.toString(), 'foo=bar&baz=quux&xyzzy=thud');
+  params.set('baz', 'updated');
+  assert.strictEqual(params.toString(), 'foo=bar&baz=updated&xyzzy=thud');
+  params.delete('foo');
+  assert.strictEqual(params.toString(), 'baz=updated&xyzzy=thud');
+  params.sort();
+  assert.strictEqual(params.toString(), 'baz=updated&xyzzy=thud');
+}
+
+{
+  const original = new URLSearchParams('a=1&b=2');
+  assert.strictEqual(original.toString(), 'a=1&b=2');
+  const copy = new URLSearchParams(original);
+  assert.strictEqual(copy.toString(), 'a=1&b=2');
+  original.append('c', '3');
+  assert.strictEqual(original.toString(), 'a=1&b=2&c=3');
+  assert.strictEqual(copy.toString(), 'a=1&b=2');
+}
+
+{
+  const params = new URLSearchParams({ foo: 'bar', baz: 1, xyzzy: false });
+  assert.strictEqual(params.get('foo'), 'bar');
+  assert.strictEqual(params.get('baz'), '1');
+  assert.strictEqual(params.get('xyzzy'), 'false');
+  assert.strictEqual(params.toString(), 'foo=bar&baz=1&xyzzy=false');
+}
+
+{
+  const params = new URLSearchParams([['foo', 'bar'], ['baz', 'quux']]);
+  assert.strictEqual(params.toString(), 'foo=bar&baz=quux');
+  assert.ok(params.has('foo', 'bar'));
+  assert.deepStrictEqual(params.getAll('foo'), ['bar']);
+}
+
+{
+  const params = new URLSearchParams('\uD83D');
+  assert.strictEqual(params.keys().next().value, '\uFFFD');
+  assert.strictEqual(params.toString(), '%EF%BF%BD=');
+}
+
+{
+  const params = new URLSearchParams('a=b+c&d=%20');
+  assert.strictEqual(params.get('a'), 'b c');
+  assert.strictEqual(params.get('d'), ' ');
+  assert.strictEqual(params.toString(), 'a=b+c&d=+');
+}
+
+{
+  // Fake percent-encoding must not be UTF-8-decoded into U+FFFD.
+  const params = new URLSearchParams('foo=%©ar&baz=%A©uux&xyzzy=%©ud');
+  assert.deepStrictEqual([...params], [
+    ['foo', '%©ar'],
+    ['baz', '%A©uux'],
+    ['xyzzy', '%©ud'],
+  ]);
+  assert.strictEqual(params.toString(), 'foo=%25%C2%A9ar&baz=%25A%C2%A9uux&xyzzy=%25%C2%A9ud');
+}
+
+{
+  const url = new URL('https://example.org/?foo=bar');
+  const params = url.searchParams;
+  assert.strictEqual(params.toString(), 'foo=bar');
+  params.append('baz', 'quux');
+  assert.strictEqual(url.search, '?foo=bar&baz=quux');
+  assert.strictEqual(params.toString(), 'foo=bar&baz=quux');
+}