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 Intel Core i5 2500K, 3570K or 4570K or above is recommended for high end emulation (e.g. PS2, Wii). This page will detail specific information for specific systems, if the above is not a viable option.

Enabling dynarec options also speed things up.

Recommended Specs

PCSX2

Minimum:

• Windows Vista / Windows 7 32-bit/64-bit
• CPU: Intel Core 2 Duo @ 3.2 GHz or better
• GPU: 8600 GT or better
• 1GB RAM (2GB if on Vista / Windows 7)

Recommended:

CPU: Intel Core i5 2500k, i5 3570K or i5 4570K

For those with DualShock 3 controllers, use the new XInput Wrapper SCP.

Dolphin

Minimum:

• 3 GHz+ dual core CPU
• DX11 GPU
• 4 GB+ RAM

Recommended:

CPU: Intel Core i5 2500k, i5 3570K or i5 4570K

See the Dolphin page for further recommendations, such as controller setups.

Mednafen PSX

It is generally recommended that people have an Intel 2.0GHz dual-core processor or better for good performance with this emulator.

Higan

bsnes - Performance, bsnes - Balanced

A 2.0 GHz Intel dual-core processor will run most games full speed.

bsnes - Accuracy ???

DeSmuMe

• Windows Vista SP2 or later
• Mac OS X: v10.6.8 Snow Leopard or later
• Linux: Any recent Linux distribution with a 2.6 kernel.
• 3.0 GHz Intel Core 2 Duo CPU or equivalent
• 2GB+ RAM

Enable the dynarec option for speedups.

PPSSPP

• Modern CPU
• OpenGL 2.1-compliant GPU

Enable the dynarec option for speedups.

CPU

MegaHertz Myth

http://www.computer-hardware-explained.com/megahertz-myth.html

Summarising the above, just because a CPU has a high clock speed (e.g. in GHz) doesn't mean that it is powerful. For example, a 4GHz Pentium IV is much, much less powerful than a 3GHz i5. Newer CPUs are always better.

AMD vs Intel

Generally, AMD CPUs perform worse than Intel CPUs for emulators specifically, due to their worse single-core performance. They will still perform well, especially if they're powerful, but will still perform worse than Intel at the same clock speeds.

Desktop vs. laptop

Laptop CPUs are typically much weaker than their desktop variants. For example, an i7 2-core laptop cpu  is weaker than a decent i3 desktop cpu. It's a common misconception that an i-whatever laptop is equal in power to i-whatever desktop

More Cores

Emulators generally only utilize 2 cores (sometimes 3), so having a hexa/octacore CPU, or one capable of hyperthreading, wont benefit you anymore than having a regular quadcore CPU.