But that's not the javascript, it's how it's used (react or whatever)

You can write terrible non-performant code in assembly and C, and perfectly speedy pages in html and javascript (say the page you're on at the moment).

You wouldn't write a new cutting edge video codec in javascript - not one you wanted to use anyway - but given that we had performant guis running on 286s at 12MHz, Javascript being 1/10th the speed of C isn't the problem.