Context for this is this blog post: https://www.peterbe.com/plog/ideal-number-of-workers-in-jest-maxWorkers

bestjest.py

import time
import subprocess

times = {}

def fmt(s):
    return f'{s:.2f}s'

def test(workers):
    cmd = f'yarn run test --maxWorkers={workers}'
    print(cmd)
    if workers not in times:
        times[workers] = []

    t0 = time.time()
    output = subprocess.run(cmd, shell=True, check=True)
    t1 = time.time()
    assert not output.returncode
    times[workers].append(t1 - t0)


import random
workers = list(range(1, 9)) * 4
random.shuffle(workers)
for worker in workers:
    test(worker)

import statistics
for workers in sorted(times):
    these = times[workers]
    print("WORKERS:", workers)
    print("BEST   :", fmt(min(these)))
    print("MEDIAN :", fmt(statistics.median(these)))
    print()

print("SORTED BY BEST TIME:")
bests = []
for workers in times:
    best = min(times[workers])
    bests.append((best, workers))
bests.sort()
for seconds, workers in bests:
    print(workers, fmt(seconds))

Run it like this:

CI=true python36 bestjest.py

...and wait.

My output became:

3 8.47s
4 8.59s
6 9.12s
5 9.18s
2 9.51s
7 10.14s
8 10.59s
1 13.80s