General Speed:
* As fast as a cheetah
* As fast as a bullet train
* As fast as lightning
* As fast as a rocket
* As fast as the wind
* As fast as a speeding car
* As fast as a racehorse
Specific Actions:
* As fast as a blink of an eye
* As fast as a waterfall
* As fast as a falling star
* As fast as a river flowing downhill
* As fast as a thought
* As fast as a magician's sleight of hand
Humorous/Figurative:
* As fast as a sloth on roller skates
* As fast as a snail in a hurricane
* As fast as a turtle on a treadmill
* As fast as a glacier in a heatwave
You can also create your own similes by thinking about things that move quickly in a particular context. For example, if you're talking about a person's typing skills, you could say "as fast as a typist on a caffeine high."