Everything depends on the game in question - there's only so much you can expect from a PSP, it's a mirracle that it even manages to emulate the N64 in the first place considering how different the two systems are and what little performance gap is between them. Try it out yourself with the games you're interested in and check results.
Here's a recorded compilation, it should give you a general idea of what kind of performance you may expect. The emulator is more optimized for some games, less for others hence the framerate spikes.
The PSP runs PS1 gaems via virtualisation, basically it has hardware capable of running PS1 gaems. And remember, PS1 gaems are running because the emulator was created by Sony. N64 is a bit of a bitch regarding emulation and is being developed by homebrewists not by a company who knows the inner workings of the system.