The term "rev up" refers to the action of accelerating the speed or intensity of something. The opposite of revving up would be to slow down or decrease the speed. Some antonyms for the term "rev up" could be to decelerate, reduce, diminish, or subside. If we are talking about performance, some antonyms could be to underperform, to idle, to stagnate, or to operate at a lower capacity. When it comes to excitement or enthusiasm, some antonyms could be to deflate, to dampen, or to wilt. Ultimately, antonyms for "rev up" will depend on the context and the particular situation.