According to Snapmaker, 120 mm/s is the endurance test they’ve made and 100 mm/s the maximum speed recommended for everyday use. It also makes sense, usual stepper motors can reach about 1000 rpm, and the old modules have 8 mm lead, thus the maximum speed would be 1000 rpm x 8 mm/rev x 60s /min = 133.33 mm/s. If you make similar number with the 20 mm lead, then you get 333.33 mm/s , which the is limited by other means.