Тестирование на Phoronix:
https://www.phoronix.com/scan.php?page=article&item=kde-gnome-wayland21&num=1
Across all the games tested both native and titles via Steam Play (Proton + DXVK), the GNOME Wayland session most often was showing the best performance and across the wide range of tests carried out came to about 4% better performance than the GNOME X.Org session. The KDE Plasma Wayland session tended to perform slightly in front of its X.Org session as well for these Linux gaming tests but there were a few games still running into problems with the KDE Wayland session.
P.S. Gnome Wayland убедительно выиграл.
Вот вам и ответ, зачем нужен Gnome и Wayland. Может и в Valve эту статью увидят или сами затестят, и переведут Steam Deck на Gnome.