From what I have gathered, the most you can find out natively in the browser about JS CPU stats, is the amount of CPU cores the client is using. Insert this in your JS file:
console.log(navigator.hardwareConcurrency)
You can then check that in the Chrome Dev Tools console.
However, you can calculate the CPU load using Node.js. Here is a step-by-step on that.
The answer on this page may also be of help in your dilemma: Javascript- Dynamically monitor CPU/memory usage