By tempo
100 BPM rap beats
100 BPM is the upper end of comfortable freestyle tempo. A bar lasts 2.4 seconds, so you get roughly a tenth of a second less per syllable than at 90 - small on paper, noticeable in the mouth.
This is a good place to move to once 90 feels easy. The extra pressure exposes the habit most beginners have of planning two bars ahead; at 100 there is no time for that, so you learn to trust the next word instead of pre-writing it.