Vacuuming has three aspects 1.Airflow 2.Suction 3.Agitation. All three are needed for good vacuuming. Airflow is how much air can be moved suction is the force it is moved and agitation is the thumping of the carpet to rattle dirt loose so the airflow and suction can pick it up. If you vacuum too fast all three aspects can’t be met at full potential. Slower vacuuming leads to less passes over the same spot so about the same amount of time with higher/better vacuuming results.