> Light doesn't change speed, so this statement confuses me. Practically speaking, it does change speed. The speed of light in a vacuum is fixed, but it varies

> Light doesn't change speed, so this statement confuses me. Practically speaking, it does change speed. The speed of light in a vacuum is fixed, but it varies based on what it's traveling through. The propagation velocity of an EM signal in RG coax cable is 80% of c*, PCB traces can be as low as 50% and somewhat surprisingly both fiber optic cable and cat6 cable are about the same, ~60-70%. I don't know if there is any good public information about the velocity factor with modern CPU transmission lines.