Hmm, in machine sliders? I see Screen Brightness under Blitter Delay.
EDIT: Found it, looks like Cheats need to be enabled for it to show up. Is the mhz clock adjustment that el_rika is specifying only possible in GroovyMame or something else?
Very stupid question, but where in MAME do I go to set CPU over/underclock? I looked around and found blitter, but I didn't see anything regarding CPU.