I have a Droid X and I have a friend with a Xoom. I have tried a Super Nintendo emulator (Snesoid) on both. The Xoom ran it perfectly with absolutely no lag that I could tell, and absolutely no input lag with the Wii remote/classic controller I was using. Can't say the same for my phone, there's slight lag and slight controller input lag (just enough for me to notice and be bothered by it). I guess the dual-core processor in the Xoom makes a big difference.
As for the Psx emulators, I've only tried psx4droid on my Droid X playing the game "Assault Rigs". I thought it ran fine, but that's probably an easy game for it to run.