New to PPSSPP.

[vsub_]

It's in the ppsspp settings=>System=>Change emulated PSP's CPU clock.
The change have effect when you set a value before starting the game.

Keep in mind that lower values can give you more speed but at the same time it can do the opposite for other games and case other problems.

About PS2/Wii...on my new pc,I can run any game I tried at native resolution at at least 3 times the normal speed

And about the cores,it depends on the program,not every program can use more than one core but pcsx2/ppsspp,dolphin can use more than one,just check the options