Yeah, I'm sorry about SNES. I'm just telling you how it is, because I know what it's like to be disappointed. Like I said in my first post, I sold my PSP once; the reason I sold it was solely because of the SNES emulation, since I too was looking for perfect handheld SNES emulation but ended up with a SNES emulator that isn't half as good as the Xbox/PC counterparts.
If you really want good SNES emulation and are getting a PSP based on that, I wouldn't get a PSP. Also, HyoImowano mentioned he's never had slowdown. When I was trying some of my favourite SNES games, mostly RPGs, the speed was OK, not full speed, but it had horrible vsync! This means that you can see the screen scrolling every so often, which gets extremely annoying, especially if you demand (almost) perfect emulation.