They take a pretty tiny percentage of my cpu time...  but I do have a PC that's much more than fast enough for Mame.
Oh yeah and I forgot, the garble sound in UMK3 didn't happen on that P4-1.5...  it happened on an athlon XP 2400 with 1gig of ram, running dual channel.  So, maybe it's just the emulation and there's nothing we can do about it?   (I guess I do have a Hercules GT-XP sound unit, which is most decidedly not a sound blaster.  Most things run alright tho.)