[alxmdev]

While 12MHz sounds weak, the heavy lifting was done by dedicated graphics hardware. Once you relieve the CPU from the burden of blitting, you're left with more than enough cycles for almost any sort of game. Even the GBA only had a 16MHz ARM! From Wikipedia:

"The SNK custom video chipset allows the system to draw sprites in vertical strips which are 16 pixels wide, and can be 16 to 512 pixels tall; it can draw up to 96 sprites per scanline for a total of 380 sprites on the screen at a time."