* Fast like a girl? This is a bit of a loaded phrase, often used in a negative way.
* A girl who is fast? This could refer to her running speed, her wit, her thinking, etc.
To help you find the right simile, can you give me more context? For example:
* What is the girl doing? (Running, thinking, talking, etc.)
* What is being compared? (Speed, intelligence, etc.)
Once I have more information, I can give you a good simile!