General Speed:
* As fast as a cheetah
* As fast as lightning
* As fast as a bullet train
* As fast as a rocket
* As fast as a race car
* As fast as the wind
* As fast as a flash of light
More specific comparisons:
* As fast as a hummingbird's wings (for rapid movement)
* As fast as a thought (for quick thinking)
* As fast as a river in flood (for rushing water)
* As fast as a runaway train (for uncontrollable speed)
The best simile for you will depend on the context of what you are describing.