console and pc cpus are different.
consoles use their cpus to do all the rendering and instructions.
emulators have to translate them to something pc can understand, and then attempt to fake those new, emulated instructions to make it work the same. while at the same time also running windows, rendering the desktop, running all background processes and also doing it all through 1-2 general use cores (java's not good with multi core cpus, and pcs have different cpu/number of cores, so you can't make it work the same for everyone), while a psp has several of them, each specialized for specific things.
tl;dr
it needs a lot of cpu power, since it tries to translate a completely foreign, alien psp language to a native pc one on the fly.
consoles use their cpus to do all the rendering and instructions.
emulators have to translate them to something pc can understand, and then attempt to fake those new, emulated instructions to make it work the same. while at the same time also running windows, rendering the desktop, running all background processes and also doing it all through 1-2 general use cores (java's not good with multi core cpus, and pcs have different cpu/number of cores, so you can't make it work the same for everyone), while a psp has several of them, each specialized for specific things.
tl;dr
it needs a lot of cpu power, since it tries to translate a completely foreign, alien psp language to a native pc one on the fly.