From what I have seen, most of the current GNOME UI is in fact just javascript. And any plugins people write for it are also javascript.

GNOME has its own interpreter, kinda how React Native does it for mobile. But performance all boils down to the layout engine. Most native UI components take shortcuts with text which is the most difficult thing to render. And the widget tree is simpler.

And there’s the whole inspector in web browser, meaning that the layout is not done once and forget. There’s various sub components still present for whatever features. Great in the browser, not great for standalone apps.