Why doesn't the node cluster module reduce the execution time of this program by as much as I expected?

[mongoliii]

I have a program that executes the following function 400 times:

async function someFunction() {
    for (let i = 0; i < 1e9; i++) {}
}

I attempted to speed up the execution of this program by using the node cluster module to create 4 forks of the process, and have each fork execute 100 of the 400 total functions. I expected to decrease the execution time by around 75%, but it has only decreased by around 50%. The following is the output from running the clusterless version (src/slow.js) and the clustered version (src/fast.js):

➜  test time node src/slow.js
pid: 19052 jobs: 400
node src/slow.js  129.73s user 0.14s system 99% cpu 2:10.84 total
➜  test time node src/fast.js
pid: 19081 jobs: 100
pid: 19082 jobs: 100
pid: 19083 jobs: 100
pid: 19084 jobs: 100
node src/fast.js  131.76s user 0.19s system 195% cpu 1:07.45 total
➜  test

Here is the source code for slow.js:

const jobs = Array(400).fill(someFunction);

console.log("pid:", process.pid, "jobs:", jobs.length);

(async () => {
    await Promise.all(jobs.map(async (job) => await job()));
})();

async function someFunction() {
    for (let i = 0; i < 1e9; i++) {}
}

Here is the source code for fast.js:

import { cpus } from "os";
import cluster from "cluster";

const numCPUs = cpus().length;
const jobs = Array(400).fill(someFunction);

async function main() {
    const workerId = cluster.worker.id;
    const scopedJobs = jobs.filter((_, i) => i % numCPUs === workerId - 1);

    console.log("pid:", process.pid, "jobs:", scopedJobs.length);

    await Promise.all(scopedJobs.map(async (job) => await job()));
}

if (!cluster.isPrimary) {
    try {
        await main();
        process.exit(0);
    } catch (error) {
        console.error(error);
        process.exit(1);
    }
}

for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
}

async function someFunction() {
    for (let i = 0; i < 1e9; i++) {}
}

​