Besides having constant practice to develop muscle memory, it'll be good to know why you want to slap fast. Reason is to know what you wanna play fast, not just any fast and senseless rhythm. Like, try to aim towards learning a Flea solo or something, god knows..
Treadmill theory: This is very simple and interesting. people who want to build stamina could use treadmills, but specifying the speed they wish to keep running at. Use a metronome or drum machine (internet can find), and set a simple groove at 100 bpm for instance, and keep drilling one groove. After youre comfortable, try improvising rhythms, to break that mental habit, or to test your mental ability to improv. But remember, one steady rhythm is usually more welcome to layman ears than a bunch of rhythms horribly put together.