"Buttery smooth" with complicated React UIs is entirely possible if you design it right. Unfortunately too many people just assume that whatever they throw at it will be fine.
Yes, but... it is difficult to do that on underpowered hardware. Modern browser engines seem to take a lot of resources to do anything at all, which gives you even less headroom before you write a single line of code. I don't think the Mach-E is equipped with a very powerful chip for the infotainment system. Since I can't find any reference to what it is, I would assume it's nothing to be proud of -- some kind of really old ARM chip, maybe.
If they equipped the Mach-E with a reasonably powerful SoC for the infotainment system, then I would agree 100% that a React-based UI could be implemented in such a way that it operates smoothly.
I've written fairly complicated React SPA UIs (certainly much more complicated than anything I've seen a car infotainment system other than, say, map rendering specifically) than run fine on relatively ancient phones and Chromebooks, so I think you're excusing just plain bad design a little too much here. I have to think they'd have the same issues regardless of platform, or if not only from using something that takes away control of the kind of things that can cause lag issues in React.
