diff --git a/lib/fetch/body.js b/lib/fetch/body.js
index 1d9f17d7e33..fd8481b796d 100644
--- a/lib/fetch/body.js
+++ b/lib/fetch/body.js
@@ -26,6 +26,8 @@ let ReadableStream = globalThis.ReadableStream
 
 /** @type {globalThis['File']} */
 const File = NativeFile ?? UndiciFile
+const textEncoder = new TextEncoder()
+const textDecoder = new TextDecoder()
 
 // https://fetch.spec.whatwg.org/#concept-bodyinit-extract
 function extractBody (object, keepalive = false) {
@@ -49,7 +51,7 @@ function extractBody (object, keepalive = false) {
     stream = new ReadableStream({
       async pull (controller) {
         controller.enqueue(
-          typeof source === 'string' ? new TextEncoder().encode(source) : source
+          typeof source === 'string' ? textEncoder.encode(source) : source
         )
         queueMicrotask(() => readableStreamClose(controller))
       },
@@ -119,7 +121,6 @@ function extractBody (object, keepalive = false) {
     // - That the content-length is calculated in advance.
     // - And that all parts are pre-encoded and ready to be sent.
 
-    const enc = new TextEncoder()
     const blobParts = []
     const rn = new Uint8Array([13, 10]) // '\r\n'
     length = 0
@@ -127,13 +128,13 @@ function extractBody (object, keepalive = false) {
 
     for (const [name, value] of object) {
       if (typeof value === 'string') {
-        const chunk = enc.encode(prefix +
+        const chunk = textEncoder.encode(prefix +
           `; name="${escape(normalizeLinefeeds(name))}"` +
           `\r\n\r\n${normalizeLinefeeds(value)}\r\n`)
         blobParts.push(chunk)
         length += chunk.byteLength
       } else {
-        const chunk = enc.encode(`${prefix}; name="${escape(normalizeLinefeeds(name))}"` +
+        const chunk = textEncoder.encode(`${prefix}; name="${escape(normalizeLinefeeds(name))}"` +
           (value.name ? `; filename="${escape(value.name)}"` : '') + '\r\n' +
           `Content-Type: ${
             value.type || 'application/octet-stream'
@@ -147,7 +148,7 @@ function extractBody (object, keepalive = false) {
       }
     }
 
-    const chunk = enc.encode(`--${boundary}--`)
+    const chunk = textEncoder.encode(`--${boundary}--`)
     blobParts.push(chunk)
     length += chunk.byteLength
     if (hasUnknownSizeValue) {
@@ -443,14 +444,16 @@ function bodyMixinMethods (instance) {
           let text = ''
           // application/x-www-form-urlencoded parser will keep the BOM.
           // https://url.spec.whatwg.org/#concept-urlencoded-parser
-          const textDecoder = new TextDecoder('utf-8', { ignoreBOM: true })
+          // Note that streaming decoder is stateful and cannot be reused
+          const streamingDecoder = new TextDecoder('utf-8', { ignoreBOM: true })
+
           for await (const chunk of consumeBody(this[kState].body)) {
             if (!isUint8Array(chunk)) {
               throw new TypeError('Expected Uint8Array chunk')
             }
-            text += textDecoder.decode(chunk, { stream: true })
+            text += streamingDecoder.decode(chunk, { stream: true })
           }
-          text += textDecoder.decode()
+          text += streamingDecoder.decode()
           entries = new URLSearchParams(text)
         } catch (err) {
           // istanbul ignore next: Unclear when new URLSearchParams can fail on a string.
@@ -565,7 +568,7 @@ function utf8DecodeBytes (buffer) {
 
   // 3. Process a queue with an instance of UTF-8’s
   //    decoder, ioQueue, output, and "replacement".
-  const output = new TextDecoder().decode(buffer)
+  const output = textDecoder.decode(buffer)
 
   // 4. Return output.
   return output
diff --git a/lib/fetch/constants.js b/lib/fetch/constants.js
index a5294a994fb..218fcbee4da 100644
--- a/lib/fetch/constants.js
+++ b/lib/fetch/constants.js
@@ -3,10 +3,12 @@
 const { MessageChannel, receiveMessageOnPort } = require('worker_threads')
 
 const corsSafeListedMethods = ['GET', 'HEAD', 'POST']
+const corsSafeListedMethodsSet = new Set(corsSafeListedMethods)
 
 const nullBodyStatus = [101, 204, 205, 304]
 
 const redirectStatus = [301, 302, 303, 307, 308]
+const redirectStatusSet = new Set(redirectStatus)
 
 // https://fetch.spec.whatwg.org/#block-bad-port
 const badPorts = [
@@ -18,6 +20,8 @@ const badPorts = [
   '10080'
 ]
 
+const badPortsSet = new Set(badPorts)
+
 // https://w3c.github.io/webappsec-referrer-policy/#referrer-policies
 const referrerPolicy = [
   '',
@@ -30,10 +34,12 @@ const referrerPolicy = [
   'strict-origin-when-cross-origin',
   'unsafe-url'
 ]
+const referrerPolicySet = new Set(referrerPolicy)
 
 const requestRedirect = ['follow', 'manual', 'error']
 
 const safeMethods = ['GET', 'HEAD', 'OPTIONS', 'TRACE']
+const safeMethodsSet = new Set(safeMethods)
 
 const requestMode = ['navigate', 'same-origin', 'no-cors', 'cors']
 
@@ -68,6 +74,7 @@ const requestDuplex = [
 
 // http://fetch.spec.whatwg.org/#forbidden-method
 const forbiddenMethods = ['CONNECT', 'TRACE', 'TRACK']
+const forbiddenMethodsSet = new Set(forbiddenMethods)
 
 const subresource = [
   'audio',
@@ -83,6 +90,7 @@ const subresource = [
   'xslt',
   ''
 ]
+const subresourceSet = new Set(subresource)
 
 /** @type {globalThis['DOMException']} */
 const DOMException = globalThis.DOMException ?? (() => {
@@ -132,5 +140,12 @@ module.exports = {
   nullBodyStatus,
   safeMethods,
   badPorts,
-  requestDuplex
+  requestDuplex,
+  subresourceSet,
+  badPortsSet,
+  redirectStatusSet,
+  corsSafeListedMethodsSet,
+  safeMethodsSet,
+  forbiddenMethodsSet,
+  referrerPolicySet
 }
diff --git a/lib/fetch/file.js b/lib/fetch/file.js
index 81bb7b2441a..3133d255ecd 100644
--- a/lib/fetch/file.js
+++ b/lib/fetch/file.js
@@ -7,6 +7,7 @@ const { isBlobLike } = require('./util')
 const { webidl } = require('./webidl')
 const { parseMIMEType, serializeAMimeType } = require('./dataURL')
 const { kEnumerableProperty } = require('../core/util')
+const encoder = new TextEncoder()
 
 class File extends Blob {
   constructor (fileBits, fileName, options = {}) {
@@ -280,7 +281,7 @@ function processBlobParts (parts, options) {
       }
 
       // 3. Append the result of UTF-8 encoding s to bytes.
-      bytes.push(new TextEncoder().encode(s))
+      bytes.push(encoder.encode(s))
     } else if (
       types.isAnyArrayBuffer(element) ||
       types.isTypedArray(element)
diff --git a/lib/fetch/index.js b/lib/fetch/index.js
index 10d84a8d192..298b3ddb27c 100644
--- a/lib/fetch/index.js
+++ b/lib/fetch/index.js
@@ -46,11 +46,11 @@ const { kState, kHeaders, kGuard, kRealm } = require('./symbols')
 const assert = require('assert')
 const { safelyExtractBody } = require('./body')
 const {
-  redirectStatus,
+  redirectStatusSet,
   nullBodyStatus,
-  safeMethods,
+  safeMethodsSet,
   requestBodyHeader,
-  subresource,
+  subresourceSet,
   DOMException
 } = require('./constants')
 const { kHeadersList } = require('../core/symbols')
@@ -62,6 +62,7 @@ const { TransformStream } = require('stream/web')
 const { getGlobalDispatcher } = require('../global')
 const { webidl } = require('./webidl')
 const { STATUS_CODES } = require('http')
+const GET_OR_HEAD = ['GET', 'HEAD']
 
 /** @type {import('buffer').resolveObjectURL} */
 let resolveObjectURL
@@ -509,7 +510,7 @@ function fetching ({
   }
 
   // 15. If request is a subresource request, then:
-  if (subresource.includes(request.destination)) {
+  if (subresourceSet.has(request.destination)) {
     // TODO
   }
 
@@ -1063,7 +1064,7 @@ async function httpFetch (fetchParams) {
   }
 
   // 8. If actualResponse’s status is a redirect status, then:
-  if (redirectStatus.includes(actualResponse.status)) {
+  if (redirectStatusSet.has(actualResponse.status)) {
     // 1. If actualResponse’s status is not 303, request’s body is not null,
     // and the connection uses HTTP/2, then user agents may, and are even
     // encouraged to, transmit an RST_STREAM frame.
@@ -1181,7 +1182,7 @@ function httpRedirectFetch (fetchParams, response) {
   if (
     ([301, 302].includes(actualResponse.status) && request.method === 'POST') ||
     (actualResponse.status === 303 &&
-      !['GET', 'HEAD'].includes(request.method))
+      !GET_OR_HEAD.includes(request.method))
   ) {
     // then:
     // 1. Set request’s method to `GET` and request’s body to null.
@@ -1465,7 +1466,7 @@ async function httpNetworkOrCacheFetch (
     // responses in httpCache, as per the "Invalidation" chapter of HTTP
     // Caching, and set storedResponse to null. [HTTP-CACHING]
     if (
-      !safeMethods.includes(httpRequest.method) &&
+      !safeMethodsSet.has(httpRequest.method) &&
       forwardResponse.status >= 200 &&
       forwardResponse.status <= 399
     ) {
@@ -2025,7 +2026,7 @@ async function httpNetworkFetch (
 
           const willFollow = request.redirect === 'follow' &&
             location &&
-            redirectStatus.includes(status)
+            redirectStatusSet.has(status)
 
           // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding
           if (request.method !== 'HEAD' && request.method !== 'CONNECT' && !nullBodyStatus.includes(status) && !willFollow) {
diff --git a/lib/fetch/request.js b/lib/fetch/request.js
index 912bd5b8c98..60e654eca11 100644
--- a/lib/fetch/request.js
+++ b/lib/fetch/request.js
@@ -13,8 +13,8 @@ const {
   makePolicyContainer
 } = require('./util')
 const {
-  forbiddenMethods,
-  corsSafeListedMethods,
+  forbiddenMethodsSet,
+  corsSafeListedMethodsSet,
   referrerPolicy,
   requestRedirect,
   requestMode,
@@ -319,7 +319,7 @@ class Request {
         throw TypeError(`'${init.method}' is not a valid HTTP method.`)
       }
 
-      if (forbiddenMethods.indexOf(method.toUpperCase()) !== -1) {
+      if (forbiddenMethodsSet.has(method.toUpperCase())) {
         throw TypeError(`'${init.method}' HTTP method is unsupported.`)
       }
 
@@ -404,7 +404,7 @@ class Request {
     if (mode === 'no-cors') {
       // 1. If this’s request’s method is not a CORS-safelisted method,
       // then throw a TypeError.
-      if (!corsSafeListedMethods.includes(request.method)) {
+      if (!corsSafeListedMethodsSet.has(request.method)) {
         throw new TypeError(
           `'${request.method} is unsupported in no-cors mode.`
         )
diff --git a/lib/fetch/response.js b/lib/fetch/response.js
index 88deb71a062..23cf55c51dc 100644
--- a/lib/fetch/response.js
+++ b/lib/fetch/response.js
@@ -14,7 +14,7 @@ const {
   isomorphicEncode
 } = require('./util')
 const {
-  redirectStatus,
+  redirectStatusSet,
   nullBodyStatus,
   DOMException
 } = require('./constants')
@@ -28,6 +28,7 @@ const assert = require('assert')
 const { types } = require('util')
 
 const ReadableStream = globalThis.ReadableStream || require('stream/web').ReadableStream
+const textEncoder = new TextEncoder('utf-8')
 
 // https://fetch.spec.whatwg.org/#response-class
 class Response {
@@ -57,7 +58,7 @@ class Response {
     }
 
     // 1. Let bytes the result of running serialize a JavaScript value to JSON bytes on data.
-    const bytes = new TextEncoder('utf-8').encode(
+    const bytes = textEncoder.encode(
       serializeJavascriptValueToJSONString(data)
     )
 
@@ -102,7 +103,7 @@ class Response {
     }
 
     // 3. If status is not a redirect status, then throw a RangeError.
-    if (!redirectStatus.includes(status)) {
+    if (!redirectStatusSet.has(status)) {
       throw new RangeError('Invalid status code ' + status)
     }
 
diff --git a/lib/fetch/util.js b/lib/fetch/util.js
index fcbba84bc9a..033fa206aed 100644
--- a/lib/fetch/util.js
+++ b/lib/fetch/util.js
@@ -1,6 +1,6 @@
 'use strict'
 
-const { redirectStatus, badPorts, referrerPolicy: referrerPolicyTokens } = require('./constants')
+const { redirectStatusSet, referrerPolicySet: referrerPolicyTokens, badPortsSet } = require('./constants')
 const { getGlobalOrigin } = require('./global')
 const { performance } = require('perf_hooks')
 const { isBlobLike, toUSVString, ReadableStreamFrom } = require('../core/util')
@@ -29,7 +29,7 @@ function responseURL (response) {
 // https://fetch.spec.whatwg.org/#concept-response-location-url
 function responseLocationURL (response, requestFragment) {
   // 1. If response’s status is not a redirect status, then return null.
-  if (!redirectStatus.includes(response.status)) {
+  if (!redirectStatusSet.has(response.status)) {
     return null
   }
 
@@ -64,7 +64,7 @@ function requestBadPort (request) {
 
   // 2. If url’s scheme is an HTTP(S) scheme and url’s port is a bad port,
   // then return blocked.
-  if (urlIsHttpHttpsScheme(url) && badPorts.includes(url.port)) {
+  if (urlIsHttpHttpsScheme(url) && badPortsSet.has(url.port)) {
     return 'blocked'
   }
 
@@ -206,7 +206,7 @@ function setRequestReferrerPolicyOnRedirect (request, actualResponse) {
     // The left-most policy is the fallback.
     for (let i = policyHeader.length; i !== 0; i--) {
       const token = policyHeader[i - 1].trim()
-      if (referrerPolicyTokens.includes(token)) {
+      if (referrerPolicyTokens.has(token)) {
         policy = token
         break
       }