Moar measurements: compositors. Since for this test the key presses are slow and there's no continuous redrawing, this should boil down to the amount of work a compositor does on screen update.

Un-vsynced X11 is obviously the fastest; thankfully work to add tearing flips to kernel and Wayland is ongoing.

Surprised to see GNOME Shell be a bit slower than raw Mutter, especially in fullscreen, since it doesn't really do much extra there. Extra surprised GNOME X11 is faster; might be noise.