diff --git a/lib/fetch/index.js b/lib/fetch/index.js
index 4471ee5d57c..b3b6e71eb79 100644
--- a/lib/fetch/index.js
+++ b/lib/fetch/index.js
@@ -9,7 +9,7 @@ const {
   filterResponse,
   makeResponse
 } = require('./response')
-const { Headers } = require('./headers')
+const { Headers, HeadersList } = require('./headers')
 const { Request, makeRequest } = require('./request')
 const zlib = require('zlib')
 const {
@@ -2075,7 +2075,7 @@ async function httpNetworkFetch (
   // 20. Return response.
   return response
 
-  async function dispatch ({ body }) {
+  function dispatch ({ body }) {
     const url = requestCurrentURL(request)
     /** @type {import('../..').Agent} */
     const agent = fetchParams.controller.dispatcher
@@ -2085,7 +2085,7 @@ async function httpNetworkFetch (
         path: url.pathname + url.search,
         origin: url.origin,
         method: request.method,
-        body: fetchParams.controller.dispatcher.isMockActive ? request.body && (request.body.source || request.body.stream) : body,
+        body: agent.isMockActive ? request.body && (request.body.source || request.body.stream) : body,
         headers: request.headersList.entries,
         maxRedirections: 0,
         upgrade: request.mode === 'websocket' ? 'websocket' : undefined
@@ -2106,59 +2106,57 @@ async function httpNetworkFetch (
           }
         },
 
-        onHeaders (status, headersList, resume, statusText) {
+        onHeaders (status, rawHeaders, resume, statusText) {
           if (status < 200) {
             return
           }
 
+          /** @type {string[]} */
           let codings = []
           let location = ''
 
-          const headers = new Headers()
+          const headersList = new HeadersList()
 
-          // For H2, the headers are a plain JS object
+          // For H2, the rawHeaders are a plain JS object
           // We distinguish between them and iterate accordingly
-          if (Array.isArray(headersList)) {
-            for (let n = 0; n < headersList.length; n += 2) {
-              const key = headersList[n + 0].toString('latin1')
-              const val = headersList[n + 1].toString('latin1')
-              if (key.toLowerCase() === 'content-encoding') {
-                // https://www.rfc-editor.org/rfc/rfc7231#section-3.1.2.1
-                // "All content-coding values are case-insensitive..."
-                codings = val.toLowerCase().split(',').map((x) => x.trim())
-              } else if (key.toLowerCase() === 'location') {
-                location = val
-              }
-
-              headers[kHeadersList].append(key, val)
+          if (Array.isArray(rawHeaders)) {
+            for (let i = 0; i < rawHeaders.length; i += 2) {
+              headersList.append(rawHeaders[i].toString('latin1'), rawHeaders[i + 1].toString('latin1'))
             }
+            const contentEncoding = headersList.get('content-encoding')
+            if (contentEncoding) {
+              // https://www.rfc-editor.org/rfc/rfc7231#section-3.1.2.1
+              // "All content-coding values are case-insensitive..."
+              codings = contentEncoding.toLowerCase().split(',').map((x) => x.trim())
+            }
+            location = headersList.get('location')
           } else {
-            const keys = Object.keys(headersList)
-            for (const key of keys) {
-              const val = headersList[key]
-              if (key.toLowerCase() === 'content-encoding') {
-                // https://www.rfc-editor.org/rfc/rfc7231#section-3.1.2.1
-                // "All content-coding values are case-insensitive..."
-                codings = val.toLowerCase().split(',').map((x) => x.trim()).reverse()
-              } else if (key.toLowerCase() === 'location') {
-                location = val
-              }
-
-              headers[kHeadersList].append(key, val)
+            const keys = Object.keys(rawHeaders)
+            for (let i = 0; i < keys.length; ++i) {
+              headersList.append(keys[i], rawHeaders[keys[i]])
+            }
+            // For H2, The header names are already in lowercase,
+            // so we can avoid the `HeadersList#get` call here.
+            const contentEncoding = rawHeaders['content-encoding']
+            if (contentEncoding) {
+              // https://www.rfc-editor.org/rfc/rfc7231#section-3.1.2.1
+              // "All content-coding values are case-insensitive..."
+              codings = contentEncoding.toLowerCase().split(',').map((x) => x.trim()).reverse()
             }
+            location = rawHeaders.location
           }
 
           this.body = new Readable({ read: resume })
 
           const decoders = []
 
-          const willFollow = request.redirect === 'follow' &&
-            location &&
+          const willFollow = location && request.redirect === 'follow' &&
             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) {
-            for (const coding of codings) {
+            for (let i = 0; i < codings.length; ++i) {
+              const coding = codings[i]
               // https://www.rfc-editor.org/rfc/rfc9112.html#section-7.2
               if (coding === 'x-gzip' || coding === 'gzip') {
                 decoders.push(zlib.createGunzip({
@@ -2183,7 +2181,7 @@ async function httpNetworkFetch (
           resolve({
             status,
             statusText,
-            headersList: headers[kHeadersList],
+            headersList,
             body: decoders.length
               ? pipeline(this.body, ...decoders, () => { })
               : this.body.on('error', () => {})
@@ -2237,24 +2235,21 @@ async function httpNetworkFetch (
           reject(error)
         },
 
-        onUpgrade (status, headersList, socket) {
+        onUpgrade (status, rawHeaders, socket) {
           if (status !== 101) {
             return
           }
 
-          const headers = new Headers()
-
-          for (let n = 0; n < headersList.length; n += 2) {
-            const key = headersList[n + 0].toString('latin1')
-            const val = headersList[n + 1].toString('latin1')
+          const headersList = new HeadersList()
 
-            headers[kHeadersList].append(key, val)
+          for (let i = 0; i < rawHeaders.length; i += 2) {
+            headersList.append(rawHeaders[i].toString('latin1'), rawHeaders[i + 1].toString('latin1'))
           }
 
           resolve({
             status,
             statusText: STATUS_CODES[status],
-            headersList: headers[kHeadersList],
+            headersList,
             socket
           })