Imagine you have a pen laying on your desk, and you push one end of the pen. You might think that the pen moves as one solid object, but in fact it takes a small amount of time for the far end of the pen to move. The pressure you are exerting on one end of the pen has to ripple down the body of the pen before the far end starts to move. The speed of this - the speed at which pressure variations travel - is by definition the speed of sound, about 5000 m/s in metal. So it might take 10us or so for the information that you have pushed one end of the pen to reach the other end of the pen. That is the maximum speed that information can travel round a mechanical system. This is turn controls how fast you can "clock" the mechanical system.

If you sent an electrical pulse down a piece of wire of similar length, it would travel at something approaching the speed of light, some 200,000 times faster.