Yes there are 2 for the mega drive
One is the gens4all which is barely playable, the other one is a pack made of the official sega emulator, but not all games runs fine and are generally worst than on a real machine
I often use the uae4all emu which is an amiga emulator. You can tell the difference from a real hardware (as i have a real amiga) but i do guess it runs 95% the speed for supported games, as it's very enjoyable
There are even the neo4all and the dreamneo cd which are a neo geo emulators. I don't have a real hardware so i can't do a comparison but games runs great honestly, so i guess it's 99% the speed
There is the aes4all which is a neo geo arcade emulator, and it runs great but most games are playable only from gdemu
There are others but i don't use them so i don't know
I hope this will help