General Synonyms:

* Swift: Implying great speed and quickness.

* Fast: Meaning moving or capable of moving at high speed.

* Quick: Suggesting promptness and agility.

* Speedy: Similar to "swift" but often with a more informal tone.

* Accelerated: Indicating a speeding up or increase in pace.

* Hurried: Implying a sense of urgency or haste.

* Brusque: Used when something happens very quickly and without warning.

More Specific Synonyms:

* Abrupt: Sudden and unexpected.

* Fleet: Very fast, especially in motion.

* Nimble: Agile and quick in movement.

* Spry: Lively and energetic, especially in movement.

* Prompt: Done without delay or hesitation.

* Immediate: Happening without any delay.

The best synonym for "rapid" will depend on the context and the specific nuance you want to convey.