What is tempo antonym?

Tempo does not have an antonym. It is a rate of speed. For clarification, tempo is often confused with a completely independent concept: rhythm.

Rhythm is also referred to as Beat. A Beat, or rhythm, is the steady, regularly repeated pattern of movement or sound in music, the pattern of repeated stressed pulses. Tempo is merely the speed of the Rhythmic beat and is expressed using descriptive Words (Allegro, Andante) or in Beats Per Minute (♩=120). Once you have established the beat/rhythm you then decide how fast or slow (tempo) you are going to play that rhythm