[httpsterio]

JS is usually what kills performance on lower end devices. If websites were only html and css, my smartphone from 2015 would still be a functional device. I have yet to have a page crash due to too poorly written html and css.

[arp242]

It depends what you're doing with the JavaScript. This is like saying "C kills performance" because you found one slow C program. JavaScript can actually improve performance in quite a few conditions (even on lower end devices) by preventing full page reloads and such, which adds more overhead in the network, HTML parsing, etc.

[httpsterio]

If we're talking about saving bandwidth, JS can be useful, although JS bundles are usually heavier than their HTML and CSS counterparts.

I have yet to see an example where rewriting the DOM would end up being lighter on devices with lower-end CPUs and RAM than doing a full page refresh. Browsers already cache static assets, generally a page refresh only takes a second or two (on reasonable connections)

[boogies]

A great example of this: compare Sourcehut to GitHub or any software forge that requires JS.

https://forgeperf.org/