are faster because they support less chipsets and roms. MAME is designed to get the game running, not necessarily at full speed. Typical lame PC Programmers mentality who can't be arsed to program properly, insisting that you need a faster processor!
MAME would be better if it had seperate 'cores' for the various hardware configurations. A lot of games out there use the same hardware engines. CAPCOMS early stuff like Strider and the like, all use the same hardware. Same with SEGA and their Model 1, 2, 3 boards. MAME is good in a way that it will support every aspect of an arcade, but relies on YOU to actually make it run at a decent speed.
As more and more arcade boards get supported, so it goes that MAME will become slower.