MAME's windows port(the one with the gui) runs like molasses on my celeron 950 mhz/256mb RAM system too.
Most of the time results like that are due to videocards not having hardware stretching support.
I have a geforce2 vidcard and if I fiddle with the options and configure mame to take advantage of the 3d card - I get a consistent 60fps framerate at 1024 X 768 reso. Except for some newer 3d games, which is largely the fault of mame not being optimized and my slow processor rather than my system exclusively.
