You should be able to find an old dos based emulator that will run fast. However it might not be pretty. Generally the reason why newer emulators are slower is because they work better. Have you tried fwNES? It's from 1998 and runs in DOS.

For genesis I would probably go for kgen. kega fusion is better but probably slower. genecyst is also worth a go.

Back when I ran Windows NT/2000 I used to boot dos off a floppy and run stuff off a zip disk. You could probably do something similar with a CD.

smf