Theres a difference between emulation which is what you are refering to and virtualization which is what the psp uses to play psx games.
Virtualization is when your device has the exact same/similar hardware needed to run what ever your running natively with out that much extra coding.
Emulation is a whole different matter where it uses software to EMULATE the components needed for the program to run.
Emulating alone takes up a huge amount of system resources and LOTS of coding so even though the 3DS may be more powerful than the psp the 3DS is not made up of mostly psone guts like the psp is.
Correct me if im wrong.
The PS3 is WAAAAAY more powerful than the ps2 but doesnt it still have trouble playing ps2 games? Thats because it uses nearly completely different hardware than the ps2 with the exception of CERTAIN early release ps3's that had both ps3 hardware and the ps2 hardware needed for the games to run.
Proof to my point
But then again we still have to wait for it to be HACKED in the first place to see whether or not it will be able to both emulate components and use them while not overloading the cpu and getting a decent framerate/speed