Modern computers are very fast, it is hard to push them to the limits using just JavaScript. 99% of performance based decisions are premature optimization. Few operations are so complex and have to be done in such large numbers that they will take to long for the user.

Benchmarks are A/B tests. What you are talking about is called a "profiler" and if you google "JavaScript Profiler" quite a few will come up (including an extension).

Modern Browsers have a "Task Manager" in "three dots" > More Tools > Task Manager.

I realized I don't know what the actual bottlenecks are on websites. Background processes adding up across tabs I assume? If someone knows please tell me/us.