Speed/Fast:
* Rapid: Indicates a very fast rate of movement.
* Swift: Suggests effortless speed and grace.
* Speedy: A more informal synonym for "fast."
* Hasty: Implies urgency or a lack of carefulness.
* Prompt: Means doing something without delay.
* Expeditious: Suggests efficiency and effectiveness in speed.
* Blithe: Means quick and carefree.
* Nimble: Refers to agility and speed in movement.
* Fleet: Describes something that moves very quickly.
* Zippy: A slang term for something that is lively and fast.
Mental/Reaction:
* Sharp: Describes a quick wit or intellect.
* Agile: Refers to mental flexibility and quick thinking.
* Alert: Means aware and responsive.
* Reactive: Suggests a quick response to something.
* Instantaneous: Happening immediately.
* Sudden: Describes something that happens quickly and unexpectedly.
Other:
* Brief: Indicates a short duration.
* Short: A more general term for something that doesn't last long.
* Ephemeral: Suggests something fleeting or temporary.
* Transient: Means lasting only a short time.
It's important to choose the word that best reflects the specific context you're using it in.