The idea of web workers as a solution for performance problem is dangerous for the web. I have 4C CPU, 6 threads max, 15 pages open in tabs, imagine each of them opening 4 web workers, because one CPU core for modern website is just not enough! madness !
I agree 100% with this (although, I would still optimise javascript where you can). My experience with any older browser has been that either loading resources or updating the DOM have slowed things down, and never the actual javascript.