What causes some plates to move faster than 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.