I forget exactly what version it happened in, sometime between .61 and .68, but the midway drivers where changed to remove a dcs hack for more perfect sound emulation. There were some other hacks removed as well. As a result you had to have a much more beefy cpu to run the games. This is NOT likely to be changed. If you were running the MK series on mame on your 2.4 Ghz PC it would run great and sound great.
Anyone is welcome to submit a patch to fix it ;-)
btw, I think the new core is a lot smoother for some games. Just not this one.