> And the "electric signal" is an electromagnetic wave- also known as light.

No, it's not. If it were, you'd have photons moving through your copper wire, which would be quite the sensation!

A moving electron does create a change in the electromagnetic field, however, so maybe that's where your confusion stems from?

More charitably, charge carriers do move much more slowly than the electric field they transmit[0], and while you’re correct that the time-varying electric field in a processor is not light (nor even a radio wave), if the chips were much larger or much higher frequency the chips and buses would risk becoming antennas and having all the problems that would bring — 5 Ghz ~= 6cm wavelength [1] ~= 3cm half-wave dipole.

[0] https://en.m.wikipedia.org/wiki/Speed_of_electricity

[1] http://www.wolframalpha.com/input/?i=c%2F5ghz

Edit: if the chips were much larger. Smaller chips can go faster without becoming antennae.

That's true but it's easy to forget that the velocity factor of that wave is significantly smaller in (say) copper than air or a vacuum.

The velocity factor of light in glass is also significantly smaller, about 70%.