Speed is a descriptor specific to motion. Speed describes the rate of change in any given motion. Speed can be defined absolutely (3 seconds) or relatively (faster, slower).

In this example, the square to the right becomes the primary figure within the sequence because of motion. The motion is a change of speed and the speed is based on a motion of changing direction (rotation, in this case). All three squares are rotating clockwise, but because the right square is rotating much faster, it becomes discontinuous.