Short answer: the controller. It's limited to a max of 225A, which at 48 volts is about 12 hp (with other losses factored in). And it only peaks around 200A for a second or two each time you upshift, so it's not delivering peak power continuously.
I actually have a video of my 7 year old nephew literally outrunning (on foot) the ForkenSwift in a flat out drag race. Takes me about 50 feet to overtake him.
