Consoles
Atari 2600 (Stella): Fullspeed.
Atari 7800 (ProSystem): Fullspeed.
Genesis (Genesis Plus GX): ~60fps, consistent.
Genesis (PicoDrive): ~60fps, same as Genesis Plus GX.
Genesis 32X (PicoDrive): To be tested.
NES (FCEUmm): 60fps, consistent (This IS the NES, after all... (^^;; )
NES (Nestopia): 55-60fps, inconsistent, recommended use FCEUmm.
PC Engine (MednaFen): ~60fps on HuCard games, ~60fps on Super CD-ROM2 games.
PlayStation (PCSX Re-ARMed): HEAVILY dependent on game, expect anywhere from 60fps in 2D-based games to 30fps in some 3D-based titles.
SNES (Snes9x 2005): ~55fps - 60fps on standard games, ~50fps on SuperFX-powered games.
SNES (Snes9x 2005+): To be tested, assumed ~50fps - 60fps on standard games. Frame drops compared to Regular '05 Snes9x are noticeable, however, on select games.
SNES (Snes9x 2010): ~40fps on standard games, requires further testing; recommend using Snes9x 2005 instead.
Sega Saturn (Yabause): Basically nonfunctional, Proof-of-Concept only.
Vectrex: ~50fps, however this is fullspeed for Vectrex.

Computer Systems
MS-DOS (DOSBox): Functional, no input allowed however.
ZX81 (81): ?
ZX Spectrum (Fuse): Full speed.

Handhelds
Atari Lynx (Handy): ~60fps
Game Boy (Gambatte/TGBDual): ~60fps.
Game Boy Advance (GPSP): ~45-50fps, generally above half-speed.
Game Boy Advance (VBA Next): ~50-60fps, below full speed, but better performing than GPSP.
Neo-Geo Pocket (Color) (MednaFen): ~60fps
Virtual Boy (MednaFen): ~25fps-30fps.
WonderSwan (Mednafen): 60fps on WonderSwan, ~45fps to 60fps for WonderSwan Color.

Arcade Systems
Final Burn Alpha: Varies by ROMset, generally better performance than MAME however.
CPS1 (FinalBurnAlpha): 60fps, consistent.
CPS2 (FinalBurnAlpha): 60fps, consistent.
Neo-Geo (FinalBurnAlpha): 60fps, consistent.
MAME (0.37b5): Varies by ROMset, generally full speed in most cases, possibly slower than FBA yet slightly more accurate(?).
MAME (0.78): Varies by ROMset, generally half-speed, if not slower; recommend using FinalBurnAlpha instead.

Engine Ports
2048 (The mobile phone game): Working.
Cave Story (NXEngine): Fullspeed.
DOOM (PrBoom): Fullspeed.
Quake (TyrQuake): With default pak0/pak1.pak, 40fps-50fps, adjustable in settings. Recommended use Rinnegatamante's VitaQuake instead.