The issue of speed input/lag in gaming is so varied and nuanced that it's hard for me to make a post about it. I think it would end up being 2 or 3 pages long to really give it a good explanation.

However it really boils down to a few things in order of importance and easy of troubleshooting.
1. Display type
-Many displays have input delay that can be turned off or reduced. CRT's are considered the best.
2. Input type
-Converter boxes like a PS2>USB box can add input delay and dropping of inputs. USB encoders like the Cthulhu are tested for very quick response, and PS/2 encoders like the J-pac and I-pac are also great.
3. Emulator/Port accuracy
-Many emulators and ports are not accurate enough to provide exact speed emulation and may be a little slower or faster than the real thing. Some have input delay that cannot be eliminated.
4. Emulator/Port settings
-Some features of emulators like Triple Buffering can add input delay. Turn them off for the quickest response. Also try running in the native resolution of the game. Some emulators run better in a window, some in full screen. You'll have to figure out what settings are best.
5. Emulator Hardware
-Most emulators run on PC's that have a lot of things going on in the background that the emulator has to taken into effect. This can lead to input delay/dropped inputs. Running your emulator on a clean, simple install of your OS of choice is best.