PSPs cpu can be clocked back to 333MHz. All those people out there using emus have already done that. Sony will also do that in GT4. They will clock the CPU back to 333MHz. As IFW said, Sony underclocked the CPU to extend battery duration. At 333MHz emus run much better, however, several emus are still slow unless you play with FPS settings, transparency and stuff. You may be able to get near Amiga speeds with much fewer FPS and a few other tweaks. Not sure how difficult or easy it is to get an Amiga or Megadrive emulated to PSP, but I think that if speed and framerate is above 85-90%, this is fine by me.
BTW, all those emulators so far are written without the psp sdk. Not sure if developing with the sdk will improve things and if yes by how much.