In music:
* Slow: If the tempo is fast, the antonym would be slow.
* Lento: A musical term for a very slow tempo.
* Adagio: A musical term for a slow tempo, but generally faster than *lento*.
In general usage:
* Pace: If something is happening at a fast tempo, the antonym could be a slow pace.
* Speed: Similar to pace, this can be used to describe the opposite of a fast tempo.
* Delay: If something is happening at a fast tempo, a delay would be the opposite.
* Pause: A complete stop or interruption of a tempo.
To give you a more specific antonym, please provide me with the context in which you're using the word "tempo."