Computer specifications
Generally, emulation runs off the CPU, with the GPU allowing for higher resolutions, AA, etc. If your CPU isn't good enough, you can't emulate a system too well. An i5 2500k or i5 3570 or above is recommended for high end emulation.
Enabling dynarec options also speed things up.
Contents
Recommended specs
PCSX2 and Dolphin
For PCSX2 and Dolphin, the recommended CPU is the Intel Core i5 2500k or the 3570k.
Mednafen PSX
2.0 Ghz Intel dualcore
Higan
Somebody said on one of the threads that bsnes-balanced was about as intense as mednafen-psx. I have no idea whether this is true because I use neither.
Dynarecs
For DeSmuME and PPSSPP, enable the dynarec option for speedups.