While I don't have the performance bottleneck numbers of React, I don't think it's about Javascript vs. WASM here.

I've seen/built some large Qt/QML applications with so much javascript and they all performed much better than your average React webapp. In fact the V8 / other browser Javascript engines also have JIT while the QML engine didn't.

Comparing QtQuick/QML + JS to HTML + JS - both GPU accelerated scenegraphs, you should get similar performance in both. But in reality it is rarely the case. I suspect it might be the whole document oriented text layout and css rules, along with React using a virtual DOM and a lot of other dependencies to give us an abstraction layer.

I'd love to know more about this from someone who did an in depth profiling of the same/similar apps on something like QtQuick vs. React.