The second, what XFCE's devs say has nothing to do with Ubuntu's and flavor's decisions, that's why I asked. I'm aware they don't plan to switch to Mir, so it's either X.org or XMir.
There must be a way to set it up otherwise, though, for the sake of testing.
Last edited by TheOne; 06-29-2013 at 04:51 PM.
If it works as I think it does (running an actual X server over Mir), and Mir is non-interfering enough (just pointing to the surface when it's a fullscreen app) it should run at around the same speed as a direct X server.