General:
* Fast
* Rapid
* Swift
* Speedy
* Hasty
* Prompt
* Agile
* Nimble
More specific:
* Sudden: for something that happens unexpectedly.
* Brief: for a short duration of time.
* Instantaneous: for something that happens immediately.
* Urgent: for something that needs immediate attention.
* Spontaneous: for something that happens without planning.
The best synonym to use depends on the specific meaning you want to convey.