I don't know a ton about it but I assume gnome-shell is where all the JS (which I gather is a huge thing in Gnome now) runs, judging from how crashy and resource-gobbling it is.
> the new KDE is surprisingly light
I had a strong aversion to KDE because I never liked how it made non-K* apps feel out-of-place, and I prefer almost none of the K* apps to other (largely GTK) alternatives, plus it used to be (talking many years ago) far heavier both in feel/performance and compile time than Gnome, but yeah, that's what I've switched to, and it performs far better. "Foreign" apps no longer feel as out-of-place as they used to and with some tweaking I don't hate the appearance.
What they've done to Gnome is so bizarre. I even kinda like the look & feel but damn, it's way too big a pig to be always-on on my machine.