Superluminal refers to a speed or velocity that is faster than the speed of light. Therefore, antonyms for superluminal would be words that denote something slower or below the speed of light. Hence, some likely antonyms could be sub-light or subluminal, sluggish, tardy, dawdling, crawling, snail-paced, sedate, hobbling, crawling, lethargic, or sluggish. Another possible antonym for superluminal could be "ordinary" or "typical" speed, which would denote average or regular speed, as opposed to something that is extraordinarily fast. Overall, antonyms for superluminal would depend on the context in which the word is being used, and the comparison being made.