fs.promises.readFile is 40% slower than fs.readFile

Version: 15.10.0
Platform: macOS
Subsystem: 11.2.1

What steps will reproduce the bug?

Run this benchmark on a 1 MB file (big.file):

const Benchmark = require("benchmark");
const fs = require("fs");
const path = require("path");
const chalk = require("chalk");
const util = require("util");
const promisifed = util.promisify(fs.readFile);

const bigFilepath = path.resolve(__dirname, "./big.file");

const suite = new Benchmark.Suite("fs");

suite
  .add(
    "fs.readFileSync",
    (defer) => {
      fs.readFileSync(bigFilepath, "utf8");
      defer.resolve();
    },
    { defer: true }
  )
  .add(
    "fs.readFile",
    (defer) => {
      fs.readFile(bigFilepath, (err, data) => {
        defer.resolve();
      });
    },
    { defer: true }
  )
  .add(
    "fs.promises.readFile",
    (defer) => {
      fs.promises.readFile(bigFilepath).then(() => {
        defer.resolve();
      });
    },
    { defer: true }
  )
  .add(
    "util.promisify(fs.readFile)",
    (defer) => {
      promisifed(bigFilepath).then(() => {
        defer.resolve();
      });
    },
    { defer: true }
  )
  .on("cycle", function (event) {
    console.log(String(event.target));
  })
  .on("complete", function () {
    console.log(
      "Fastest is " + chalk.green(this.filter("fastest").map("name"))
    );
    console.log("Slowest is " + chalk.red(this.filter("slowest").map("name")));
  })
  .run({ defer: true });

To create a 1 MB file (~40% slower):

dd if=/dev/zero of=big.file count=1024 bs=1024

To create a 20 KB file (~55% slower):

dd if=/dev/zero of=big.file count=20 bs=1024

How often does it reproduce? Is there a required condition?

Always.

What is the expected behavior?

fs.promises.readFile should perform similarly to fs.readFile

What do you see instead?

Additional information

I suspect the cause is right here: https://github.com/nodejs/node/blob/master/lib/internal/fs/promises.js#L319-L339

Instead of creating a new Buffer for each chunk, it could allocate a single Buffer and write to that buffer. I don't think Buffer.concat or temporary arrays are necessary.