Indeed, but web is not computationally expensive. Even old computers like my laptop with i3-6157u CPU handle web just fine.

What do you mean by that?

Firefox and Chrome both have gpu hardware acceleration, and different threads/processes for UI and webpages.

Javascript is parsed and executed on one core.

If you have just one tab, with a simple webpage with no iframe, yes. But this use-case is handled even by a first-gen Raspberry Pi.

Once you start having several tas open, or web pages with different iframes, modern browsers put those in different processes.