Conveyor belts are slow. They realized long ago that you can't accelerate a standing human too fast and that stepping on to a conveyor belt takes you to max speed nearly instantly. Any faster and people need a running start to jump on.
Somewhere I saw a prototype of a multi-belt system as well. You get on a shorter one at normal speed, and you can either transition to one moving twice as fast as the first, or you can step off completely. The speed difference from stationary to slow is the same as from slow to fast - you just accelerate by shifting belts.