Properly emulating a SNES (like a bsnes approach) requires > 3 Ghz Core2 processor class integer performance. Many popular emulators do not emulate to a cycle or sub-cycle exact accuracy. Many fast emulators have graphics glitches that are often overlooked. Many emulators are incomplete in their sound emulation, simplifying the sound model to eat less cpu but introducing subtle bugs in the process. The problem is that emulators are single threaded because of the fundamental problems of cycle accurate sychronization. Consequently using more than 1 cpu to speed up emulation often gains very little. Cycle accurate emulation of the NES requires ~1.4 Ghz Pentium 4 processor, far higher than most people would assume. PS2, Xbox & Gamecube simply do not have the single threaded performance to emulate the 25 year old NES with cycle exact accuracy. So people port fast emulators that are incomplete models of the hardware. The perfect example of this is UltraHLE where its possible to play a N64 game like goldeneye on a 233Mhz Pentium 2+3dfx Voodoo2 class hardware. UltraHLE takes the approach to emulate as little as possible the underlying hardware but instead translate C code directly and translate RSP&RDP calls to Glide API calls. The problem with this approach is that it only works in specific cases, the rendering process done on the RSP&RDP almost never exactly matches to Voodoo behavior. Even today the video rendering of N64 on more advanced emulators is still not close to perfect, lots of video bugs to be had.
So in short sub-cycle exact emulation of the following
NES ~>1.4Ghz Pentium 4
SNES ~>3Ghz Core2 solo
N64 ? unknown much higher than SNES, almost all emulators have incomplete RDP&RSP emulation
Dreamcast >2.6 Ghz Pentium 4+ incomplete video emulation
PS2 current emulators require > 3.6Ghz Core2 Duo, many games have lots of video bugs, complete emulation still many years away
Gamecube incomplete, good luck, requirements much higher than any modern hardware can provide
Xbox emulation good luck(whats done on the xbox360 is hackwork more like ultrahle approach than bsnes)
Virtual Console most emulation still use incomplete model