What causes some plates to move faster than others? Why do some tectonic plates move faster than others?  Is it due to friction between the plates, the driving forces on the plates, or both?  If it is driving forces, what makes the driving forces on some plates stronger than on others?
The driving forces of plate tectonics has long been debated. Several mechanisms have been proposed, mainly ridge push, mantle drag, and slab pull (see this page from the British Geological Survey). However, there is now a good agreement that slab pull is the main driving force of plate tectonics. It means that it is the subducting plate which controls the velocity of the plate's movement. And the rate at which a plate sinks depends mostly on its age/temperature/density: older plates are cooler/denser, thus they sink at a higher velocity than younger plates.