There is an option to underclock or overclock the SH4 processor of the Dreamcast. Underclocking it sometimes allows for faster framerates, while overclocking generally makes it go slower. This is not definite, it is dependent on the the processor dependancy of the game and your computer's processor, I think all it does is bypass the nop operations when underclocking and adds nop when overclocking. Oh, the controls for this are + and -. In the end, don't expect any major improvement when underclocking, and when a game already uses the SH4 processor to a high degree, underclocking makes the game go slower.