The speed of music is its tempo (the rate at which the basic, regular beats of the meter follow each other)
It’s expressed by the metronome, a device which plays regular beats
Tempo indications are in Italian! they include… adagio (slow), andante (slower, but not too slow), moderato (moderate), allegretto (faster, but not too fast), allegro (fast), presto (very fast)
Less common tempo indications include… largo, lento, grave (slow, very slow), larghetto (faster than largo), andantino (faster than andante), vivace, vivo (lively), molto allegro (faster than allegro), prestissimo (VERY fast)
Use the English meaning to determine what it means overall- think of vivace as vivacious and allegro as cheerful, this makes the meaning more defined
Sometimes the speed is used to indicate certain sections