SF 12/20 is stock speed, just undervolted. SF 12/19 is overclocked and undervolted with a few minor issues to date. I found 12/20 to be pretty quick but after 48 hours battery life dropped off considerably. I've sense switched back to my old standby of 1.54. Like 12/20, it is stock speed and undervolted, just not as much. Rock solid, very snappy, and great battery.