There are two kinds of "frames per second".
The first is the virtual FPS. If this is running at 60/60 then the emulator is running at full speed. The second is the real internal FPS, which is whatever the game actually ran at. Most consoles use the following internal FPS based on region:
- NTSC (USA, Japan): 60FPS
- PAL (Europe, Australia): 50FPS
Note that in more recent consoles, such as the GameCube/Wii, PAL can also run in 60FPS.
With the move to 3D, however, it took much more CPU power to run games at 60FPS. So few games actually ran at that. Most ran at 30FPS, and some at 20FPS, or even lower. Zelda Ocarina of Time for instance runs at 20FPS. Star Fox ran below that.
If you are experiencing slowdown, it may be that the emulation is too intense for your system. When this happens, both the video and the sound experience slowdown. To counter act this, you can enable speed hacks, use a less intensive emulator/settings or get a new CPU. However, the slowdown may be part of the original game, and will be emulated. As such there is nothing that can be really done, short of reprogramming the game itself or emulating an overclock. When this slowdown occurs, the video will slowdown but the audio will often continue at full speed. An example of a game with real slowdown like this is Metal Gear Solid (PS1).
A way to counter real internal slowdown is to overclock the cpu of the emulated system. This of course requires a faster CPU to run the emulator of course. It may result in a smoother frame rate, or unintended effects such as speeding up the entire game. It is very game and system depdentent. Emulators that support this:
Using speedhacks can affect both types of FPS in emulators. Most of the time it only affects the emulator's speed though, to help achieve 60/60FPS. Some speedhacks, however, can lower the internal FPS to raise the emulator's FPS; an example of this is in PCSX2, known as EE cycle stealing, which reduces the clock speed of the virtual CPU used, the Emotion Engine. No speedhacks raise the internal FPS, as this should always be at maximum unless altered as previously mentioned.