It isn't. Not for what people are trying to make with it.
Case in point: https://krausest.github.io/js-framework-benchmark/2023/table...
The benchmark creates 1000 rows that look like this:
<tr>
<td><a onclick={select this row}>random short text</a></td>
<td><a onclick={remove this row}><span /></a></td>
<td></td>
</tr>
The fastest of the fastest attempts to do this takes 36 milliseconds to render. For what is essentially static markup with zero complex logic or complex interactions.
In comparison: 1000 actors with complex interaction and behaviour, complex animations and lighting takes 4 milliseconds to render (in total, significanly more than the measley 5-6k static elements on the page): https://youtu.be/kXd0VDZDSks?si=SswSZLNFlRd7adsM&t=586 (at 9:46)
I'm not saying everything should be Unreal Engine. But the web is on the lowest of the lowest end of the spectrum when it comes to performance.