I haven't see any project which feels fast which are written in React. But most important thing for me which is broken (because it is too difficult to catch all edge cases) in all the JS frontends is the broken navigation (browser's back and forward almost never work as expected, bookmarking links are broken because state is in JS etc.)
This can definitely be fixed (it involves making sure the relevant operations in the app manipulate `window.history` and either indicating location state in the browser via the hash portion of the URL or building the server to work hand-in-glove with subpaths), but it requires more work than the default navigation one gets with a multi-page app.
I've seen good frameworks for managing this but I agree that developers tend to forget it.
