Plenty of GTK3 software is backed by Python, so switching to GJS should actually improve responsiveness. GTK can certainly be used without declarative GUI and stylesheets, but most programs seem to use both to design the GUI already.

I think React + GJS is a good fit. I don't think it's a good idea to replicate this to other GUI frameworks (Qt, Win32, etc.), but GJS already has most of the components necessary built right into it. Perhaps there are more light-weight alternatives, but this specific combination is a win-win in my opinion.