Sorry but I have tried wayland multiple times (most recent ~12mo ago) and this has never been true in my experience. At least on my nvidia card. And the wayland people have been saying stuff like "oh we know better performance is possible, we just need to implement x, y, and z" for many years.
No, most Wayland compositors can already skip composition and pass the window contents directly to the screen in case of displaying just a single fullscreen window. There is also work ongoing on utilizing hardware planes when possible, to skip composition and improve performance in other cases as well - going beyond what was ever possible on X. Grandparent comment is simply wrong.