http: avoid dictionary-mode objects in responses

benchmark/fixtures/simple-http-server.js

@@ -114,6 +114,12 @@ module.exports = http.createServer((req, res) => {
       else
         res.writeHead(status, { 'Content-Length': len.toString() });
       break;
+    case 'setHeaderImplicit':
+      // Let end() derive the message framing: no Content-Length or
+      // Transfer-Encoding header and no writeHead() call.
+      res.statusCode = status;
+      res.setHeader('Content-Type', 'text/plain');
+      break;
     default:
       if (chunkedEnc) {
         res.writeHead(status, {

benchmark/http/set-header.js

@@ -2,7 +2,7 @@
 const common = require('../common.js');
 
 const bench = common.createBenchmark(main, {
-  res: ['normal', 'setHeader', 'setHeaderWH'],
+  res: ['normal', 'setHeader', 'setHeaderWH', 'setHeaderImplicit'],
   duration: 5,
 });
 
@@ -15,6 +15,7 @@ const c = 50;
 // normal: writeHead(status, {...})
 // setHeader: statusCode = status, setHeader(...) x2
 // setHeaderWH: setHeader(...), writeHead(status, ...)
+// setHeaderImplicit: statusCode = status, setHeader(...) x1, framing left to end()
 function main({ res, duration }) {
   const server = require('../fixtures/simple-http-server.js')
   .listen(0)

lib/_http_outgoing.js

@@ -87,6 +87,15 @@ let debug = require('internal/util/debuglog').debuglog('http', (fn) => {
 });
 
 const kCorked = Symbol('corked');
+
+// Container for the headers set through `setHeader()`, keyed by the
+// lower-cased header name. It has no enumerable or inherited properties, like
+// a `{ __proto__: null }` literal, but instances are fast-mode objects: V8
+// creates null-prototype literals in dictionary mode, which costs a hash table
+// allocation per response and slower property access for every header.
+class OutgoingHeaders {}
+ObjectSetPrototypeOf(OutgoingHeaders.prototype, null);
+delete OutgoingHeaders.prototype.constructor;
 const kSocket = Symbol('kSocket');
 const kChunkedBuffer = Symbol('kChunkedBuffer');
 const kChunkedLength = Symbol('kChunkedLength');
@@ -124,6 +133,18 @@ function OutgoingMessage(options) {
     return deprecateInstantiation(OutgoingMessage, 'DEP0195', options);
   }
 
+  // Preset the listener table with the events that are commonly listened
+  // for, mirroring streams. The presence of the properties keeps `_events`
+  // in fast mode: the first `on()` call would otherwise have to allocate a
+  // dictionary for the `{ __proto__: null }` object created by EventEmitter.
+  this._events ??= {
+    close: undefined,
+    error: undefined,
+    finish: undefined,
+    prefinish: undefined,
+    drain: undefined,
+  };
+
   Stream.call(this);
 
   // Queue that holds all currently pending data, until the response will be
@@ -805,7 +826,7 @@ OutgoingMessage.prototype.setHeader = function setHeader(name, value) {
 
   let headers = this[kOutHeaders];
   if (headers === null)
-    this[kOutHeaders] = headers = { __proto__: null };
+    this[kOutHeaders] = headers = new OutgoingHeaders();
 
   headers[name.toLowerCase()] = [name, value];
   return this;