google tachometer

https://github.com/google/tachometer

tachometer is a tool for running benchmarks in web browsers. It uses repeated sampling and statistics to reliably identify even tiny differences in runtime.

Even if you run the same JavaScript, on the same browser, on the same machine, on the same day, you'll still get a different result every time. But if you take enough repeated samples and apply the right statistics, you can reliably identify even tiny differences in runtime.