> What's particularly heavy in these gnome tools?

Nothing. On my phone there's 770MB RAM consumed by the entire system with Phosh running right now. The rest is free to be eaten by the browser.

If anything, it's GTK that can be rather inefficient when it comes to rendering (both GTK3 and GTK4, but for different reasons). There are some tricks that phoc (Phosh's compositor) could still learn too, but it will get there.