It is a reasonable big deal in the niche of computation intensive applications.
It would be an huge deal or revolution if it could perform faster on plain DOM manipulation. By now, plain Javascript is faster than WASM [0].
That's because all WASM's DOM manipulation must go through the Javascript engine and many browsers do JIT compilation on Javascript.
Edit: be aware that the article has a more interesting discussion on WASM on the server side, as a replacement of technologies such as Docker/containers, Java Virtual Machine and .Net.
[0] https://krausest.github.io/js-framework-benchmark/2023/table...
Unless you don't use the DOM at all for UI for web apps: https://news.ycombinator.com/item?id=34612696
It would enable any language to simply render their own UI and handle things like accessibility with their own accessibility based DOM that just handles the accessibility parts rather than the entire logic of the app. I believe this is what Flutter does.