Can't explain this Node clustering behavior - node.js

Question

I'm learning about threads and how they interact with Node's native cluster module. I saw some behavior I can't explain that I'd like some help understanding.
My code:
process.env.UV_THREADPOOL_SIZE = 1;
const cluster = require('cluster');
if (cluster.isMaster) {
cluster.fork();
} else {
const crypto = require('crypto');
const express = require('express');
const app = express();
app.get('/', (req, res) => {
crypto.pbkdf2('a', 'b', 100000, 512, 'sha512', () => {
res.send('Hi there');
});
});
app.listen(3000);
}
I benchmarked this code with one request using apache benchmark.
ab -c 1 -n 1 localhost:3000/ yielded these connection times
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 0.0 0 0
Processing: 605 605 0.0 605 605
Waiting: 605 605 0.0 605 605
Total: 605 605 0.0 605 605
So far so good. I then ran ab -c 2 -n 2 localhost:3000/ (doubling the number of calls from the benchmark). I expected the total time to double since I limited the libuv thread pool to one thread per child process and I only started one child process. But nothing really changed. Here's those results.
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 0.1 0 0
Processing: 608 610 3.2 612 612
Waiting: 607 610 3.2 612 612
Total: 608 610 3.3 612 612
For extra info, when I further increase the number of calls with ab -c 3 -n 3 localhost:3000/, I start to see a slow down.
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 0.0 0 0
Processing: 599 814 352.5 922 1221
Waiting: 599 814 352.5 922 1221
Total: 599 815 352.5 922 1221
I'm running all this on a quadcore mac using Node v14.13.1.
tldr: how did my benchmark not use up all my threads? I forked one child process with one thread in its libuv pool - so the one call in my benchmark should have been all it could handle without taking longer. And yet the second test (the one that doubled the amount of calls) took the same amount of time as the benchmark.