Here's another example to help you understand a little better why the speed difference:
PacMan - Ran in the arcades on a Motorola Z80 @ 3.072MHz. This is about equivalent to the IBM PC XT, which used an Intel 8088 (8086?) CPU at 4.77 Mhz. Slowest PC to play it in MAME would be around a Pentium I 150 Mhz (guesstimating). Not only is that about 30 times faster, but it also five generations of chips faster - 8088, 80286, 80386, 80486, Pentium, Pentium II, Pentium III, Pentium IV, etc.
Modern Games - SF Rush - R5000 (little) @ 200MHz, also a TMS32031 @ 33.8688MHz. The main processor on this board is a little faster than the Pentium PC I mentioned above. I've seen estimates that it would take a Pentium 4 at about 10Ghz to run it properly. More realistically, 30x speed would be about 6 Ghz, so probably in about 4 years, on a Pentium 8 at 6 Ghz, it should be playable.
BTW - Really modern games -
Afterburner 3 (unemulated) runs on a Intel Pentium 4 3.0G HT (800Mhz FSB - 1MB L2 Cache) with a 256M graphics card and a GIG of Ram. It will probably be about 10-12 years before computers will be fast enough to play emulations of this (and maybe longer to white the code).
Bottom line is - 95% of the games in MAME will play decently on about a 1.5 - 2.0 Ghz processor, and the other 5% are unplayable on hardware at triple the price, so . . .