Is the speed of signal transport via electricity as fast as light? Let us assume a time synchronization system that comprises a sender and a receiver. The sender generates and sends an encoded signal which presents the current time to the receiver periodically, and the receiver calibrates its clock according to this signal. Is the speed of signal transport via electricity as fast as light?
If it is,  does it mean no matter which media we use, copper or fiber, even air(WiFi), the time lag between the sender and the receiver is identical theoretically(ignore interference)?
 A: The propagation rate of a electrical signal along a wire depends on how much capacitance and inductance it exhibits on a per-foot basis. These parameters vary according to the diameter of the wire, its construction (parallel vs. coaxial), the proximity of other wires, and the type and thickness of insulation it is coated with (if any). The resulting propagation speed will always be significantly less than that of light in a vacuum and hence will make for example a wire antenna's electrical length different from its physical length. Antenna designers must measure the propagation speed of signals in samples of the wires they use to account for this effect- or else their antennas will not resonate properly at the design frequency.
A: Electrical signals propagate as electromagnetic waves. If  conductors are present, the waves occupy the space around the conductors. The geometry of the conductors is designed to guide the wave.
The nature of the conductor guiding the wave (copper, aluminum, gold, ...) has little influence. The medium in which the wave propagates (polyethylene, PTFE, air, ...) matters: the speed of light in material media is reduced.
The geometry of the conductor matters: waves on straight vacuum-insulated coaxial or balanced lines move at the vacuum speed of light. Other geometries slow the waves down.
Fibers are, of course, material, and therefore the speed of light is slower in them. Electromagnetic modes in fibers are also somewhat slower than free-space modes.
