The whole architecture of the iPhone/iPod Touch is totally different than that of the DS or PSP.
A computer that is a couple dozen times faster than a DS cannot even emulate it that well. The whole structure of the processing needs to be translated. That is why in emulated DS games there are artifacts in the gameplay and major slowdowns/choppiness.
Also, about the iPhone being faster -
Sure, the iPhone is faster, but it cannot replicate graphics even close to the PSP.
The PSP's clock speed is indeed slower (333mhz) compared to the 667mhz IIRC?
But you have to take into account all the background processes, the game quality, and the graph