The similar word of 'sluggish'-

Sluggish (Adjective):
Meaning:
1. Slow-moving or inactive.
2. Lacking energy or alertness.

SYNONYMS: -
- lethargic, listless, lacking in energy, unenergetic, lifeless, inert, inactive, slow, torpid, dull.

ANTONYMS: -
- vigorous, energetic, active, animated.

Source: Oxford Dictionary.