I doubt there is very much you can do. I'm not sure what your expectations and experience with emulators are and I don't know exactly what app and game you are trying to run, but it's very often the case that emulators don't perform as well as you may expect. Especially the more complex consoles. Despite the fact that the PSX is a good age now, it still not a trivial task to emulate it on a device like the iPad, and emulation by its very nature makes things slower than it would have originally been by several orders of magnitude. The platform you are running the emulator on therefore has to have enough grunt to compensate for this. You don't mention which iPad you have, but it could also be that the emulator has not been tuned for your device model yet.
If you've closed all the apps on the iPad and configured any obvious settings for the emulator, then it's probably running as fast as it can. It could well be there just aren't enough resources to emulate the game at an acceptable speed.
That said, if you are using the legitimate paid version of PSX4ALL, performance should be OK depending on the game.