General antonyms:
* Energetic
* Vigorous
* Lively
* Active
* Dynamic
* Spirited
* Brisk
* Fast
* Quick
* Agile
More specific antonyms:
* Alert (referring to mental state)
* Productive (referring to work)
* Efficient (referring to performance)
* Rapid (referring to speed)
* Nimble (referring to movement)
The best antonym to use will depend on the specific context in which you are using the word "sluggish."