By tempo
120 BPM rap beats
120 BPM is where rap starts overlapping with dance tempo. Most rappers at this speed switch to half-time - counting every other beat as the downbeat - which gives you the phrasing room of 60 BPM over a beat that still drives.
Practise both ways. Rapping straight at 120 trains articulation; rapping half-time at 120 trains placement, because you have four subdivisions to land on instead of two.
A bar at 120 BPM lasts exactly two seconds. That is a useful number to hold on to, because it means sixteen bars is thirty-two seconds and a full verse is close to a minute - you can time a performance without a stopwatch once the arithmetic is automatic.
The trap at this tempo is over-filling. Two seconds feels short, so beginners cram syllables in and end up rushing the last word of every line to make it fit. The fix is to write to the half-time count first: get comfortable landing one strong word every four seconds, then start adding between them. Rushing is almost always a symptom of having planned too many words, not of the beat being fast.
If you are coming from 90 or 100 BPM, do not jump straight here. Spend a week at 100 first; the gap between 100 and 120 is bigger in the mouth than it looks on paper.