welcome! not sure the performance of the emulator on the iphone, but a thing to remember is that the PSP is capable of running PS1 games as well under pspboot isos so its not just psp games, but also playstation games to consider.
personally, if you're after something quick, try lumines a fairly addictive puzzle game. Perhaps the patapon series.
I believe outrun on the PSP is pretty good as well as wipeout on the system.
There are of course RPGs on the psp like disagea series, phantom brave, tactics orge reborn etc.
of course we couldn't forget suggesting GTA vice city stories. if you want, Liberty city stories as well, but the map decision imo isn't as great and isn't as refined as VCS was (visually or mechanically).