Back in like 1999, the computer I had at the time which was already a few years old, could run the DOS version of MagicEngine at almost full speed.
Nowadays I use a Macintosh and an emulator called TGEmu. There are a few games where the sound doesn't work, but the emulator is very fast even on my aging G4. It will do full screen video in the proper aspect ratio and can simulate scanlines. Even in this mode, it has no problem running at full speed. It is very configurable. Best of all, it is free.
But the only thing emulators are good for anyway are trying games before you decide whether or not to buy them. Anyone who uses an emulator as their gaming "console" is clearly a loser.