Do not want. If the server side / backend is slow, it's still going to be slow when handling API requests from JavaScript (of which there probably will be more per page than just a single HTML request now), all that's going to happen is that the heavy mass of React will also make the front-end slow in addition to that.

My main phone is still fairly modern and it already struggles with the giant SPA that the new Dex has become (in contrast to Dynasty or AO3 still working very well even on the ancient iphone that I still use for funz)... I'm sure things like fancier zooming could still be done in JS without overdoing it.