Except GNOME rewrote GNOME Shell in JavaScript. I don't think GTK3 requires GJS, so I'm not sure why it uses more RAM than GTK2, maybe due to CSS? I know it's still Cairo-based, not GPU-accelerated or DOM-like.
I can't say for sure what the deal is with GTK3, but GTK4 apps are GPU accelerated and should be faster. Though it may come at a cost of using more GPU memory, depending on the app.