I think it's more of a personal perception.
For me, I feel the balls with 2.5 - 3.0 speed are pretty much the same while those with 3.5 - 4.0 are relatively much slower, almost like there's a sudden "jump-over", a marked difference (gap) between the two groups. And of course, the balls with 5.0 or slower speed belong to a completely different league...