Still, it's fast enough that Rust frameworks can compete with the fastest JS frameworks on UI benchmarks [1]

Imo bundle size is a bigger issue.

[1] https://youtube.com/watch?v=4KtotxNAwME

Yes, that contributes to the "if it actually worked" sentiment.

It doesn't have to be slow, if the browser exposed C-like ABI for DOM access and web APIs like LocalStorage, the FileSystem API, ServiceWorker, etc - Rust bindings can be made and that boundary could be well optimised.

All we need is;

<script type="application/wasm" src="./main.wasm"></script>

The rest is just browser optimisations.