Here's how I understand the situation: when an electrical current is running through a wire, the individual electrons are moving very slowly. The reason that a light bulb turns on so quickly after the switch is closed, then, is because as the electrons near the switch start moving, they push on the electrons nearby, which push on the electrons nearby, and so on: a "wave" -- or a disturbance of some sort at least -- is created, which reaches the light bulb almost instantaneously. But don't disturbances of this sort travel at the speed of sound (in the material)? Why does this disturbance travel so much faster? I don't think it's because the electrons are lighter: the difference in speed seems too great.