Depends on what you wish to play I guess. I've got a mid range (AMD Athlon64 2.2GHZ) and can get quite playable framerates in the Nipon Ichi games(Disgaea, Makai kingdom, La Pucelle Tactics, Phantom Brave). Maybe around 85% speed or so.
Kind of hard to test the 3D stuff since those games tend to use heavy amounts of FMV and from what I've seen this emulator has problems running video.
OK just tried Final Fantasy X. In windowed mode (320X240) I was hitting around 15FPS when the whole group was on screen around the fire in the opening...about 35-40 FPS when it was just the close up shots of Tidus and Yuna. It crash looped when he was talking on the mountain (or it was just going so slow it never got around to updating to the next line) so I have no idea what it's like in game. The intro bit had severe graphics errors on all the surroundings as well so not sure if this game could be suffering from behind the scenes stuff that could be adding to the slowdown. Speed was a little better in 640X480 75htz fullscreen.
I suppose if you're playing a simple 3D game with minimal characters on screen at a time you may be able to get about half speed.