General:
* Swift as a shooting star
* Swift as a cheetah on the savanna
* Swift as a falcon in flight
* Swift as a thought
* Swift as the wind
More specific:
* Swift as a bolt of lightning (emphasizing speed and force)
* Swift as a whisper in the breeze (emphasizing quiet and subtle movement)
* Swift as a river rushing to the sea (emphasizing continuous and powerful motion)
* Swift as a hummingbird's wings (emphasizing rapid and precise movements)
The best simile for you will depend on the specific context and what you want to emphasize about the swiftness.