If electrons move slowly in an electrical circuit then what signal or energy is it that travels at the speed of light?
It is changes in the fields inside and around the wire that travel at the speed of light. Imagine the wire as a hollow tube full of electrons. When there is no current the electrons are all sitting there, repelling each other, but since there is nowhere to go, they just sit still.

When an electron at the back of the wire starts getting pushed (by a battery for example), it gets closer to its neighbors in front, which then pushes them forward. These electrons start to move, which then causes them to start pushing on their neighbors. Eventually all the electrons are moving down the wire.

The speed with which the electrons hear about their neighbors moving determines how quickly the signal travels down the wire. Nothing about this process actually requires the electrons to actually go anywhere quickly, just that their neighbors feel changes quickly.

This is all a huge simplification, in reality there is a continuous electric field, and there is a magnetic field generated around the wire, and of course the wire isn't hollow, but this picture can help for getting the concept.