In theory, electrical signals move at the speed of light. Cables only slow them down. The ratio of actual speed to the speed of light is known as the velocity factor, or Velocity of Propagation (VOP), expressed as a percentage of the speed of light in free space.
This slowing effect is almost entirely caused by the dielectric material; in coaxial cables, the insulation between the shield and the center conductor. For a closed-cell foam dielectric, for example, the VOP may approach 90%, meaning that a signal will travel at 90% of the speed of light. For solid Teflon®, the VOP is typically about 70%.