There are a few factors that can contribute to wheelchair speed. When speaking about electric wheelchairs you need to account for the weight of the chair, the torque that the motor is able to put off, how large and strong the battery is, and the run conditions you are facing.
While some electric chairs use two batteries that are able to produce more power along with a motor that produces the torque needed to move faster than others. While going fast might sound fun for some there are speeds that should be followed to keep the wheelchair in the safest possible operating status while using it.