Anecdotally I run X11 on several Sun workstations (Motorola 680xx), a Dec Alpha (some RISC), HP X terminals, etc. All of them were reasonably fast or at least not any slower than the Windows and Mac boxes of the time (1990/95.)

We played videogames on them. Does anybody remember Netrek, Xtanks, and a F16 vs MIG flight simulator which I can't remember the name of?