KDE has more manpower, and while it still has far too many bugs to be my daily driver, I only have tons of respect for the galaxy-brain maniacs who work on it.
XFCE doesn't have such a big team, and they are careful to maintain stability, so they are going to work slower. I accept this.
The reasons I _personally_ am convinced Wayland is bad has nothing to do with XFCE support, really.
I can excuse XFCE for being slow. However that is very different from the claim that because XFCE is a bit slow that Wayland is in general bad.