There are special cases where this won't work. For example I'm working on audio processing software and I can't test it in a VM because audio latency in a VM is too big whereas an older and slower computer might have slow disk access, etc. but audio latency is still near realtime.

Are there any special kinds of emulation that you could use for this purpose? I mean, there are DOS, gameboy emulators out there, it would be interesting if there were emulators out there for PCs with a particular hardware configuration which would emulate slow disk access and so on.