Is the speed of electrical signals always constant within a CPU?
Electromagnetic waves travel at a fraction of the speed of light, depending on the dielectric constant of the medium they are propagating through. If you had ideal components (lossless traces, circuit elements that switch instantaneously, power supplies with no internal impedance, no noise in your signal etc.) maybe you could approach these speeds. The reality is the non-idealities of the circuits will dominate the performance. Signals in a modern processor will be limited by the node capacitance, switching speeds of internal transistors/mosfets and voltage levels necessary to register high/low signals. These metrics will dictate the rise/fall times necessary to transmit information. 

Yes, this introduces lots of complexity but that is why there are very smart people with PhDs and entire engineering teams necessary to design integrated circuits. Monitoring these metrics across process variation, temperature variation and voltage variation is part of a robust integrated circuit design and there is no getting away from that complexity.