[nevermore24]

You can only optimize at your current level of abstraction. Writing JavaScript? Have fun, there's a floor to your performance, because you're on the 28th floor of the abstraction skyscraper, and you can only go up. Part of the issue is that developers don't know any better, the other part is that everyone suddenly decided that four virtual machines crammed inside each other like a matryoshka doll is the only valid way to write software anymore.

[redox99]

JS by itself isn't that bad. You could calculate pi digits reasonably fast using JS.

The problem usually is: really poor code that is blocking, triggers a million rerenders for every interaction, insane bloat, or just the absolute massive amount of abstractions through packages. Plus HTML + CSS is really slow.