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

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.