How fast is DC current/voltage?
First, as stated, DC carries no information. Switching DC on or off creates a step function, i.e. a waveform with harmonics that may go into the RF. A &quot;perfect&quot; (theoretical) step waveform would have harmonics through radio frequencies (RF), infrared radiation (IR), visible light and all the way to gamma rays... but since decreasing the wavelength increases energy of the minimal photon, that is actually not possible. However, RF harmonics can be heard as a click on a nearby AM radio when opening or closing a DC circuit.
So your question should be how fast a specific frequency propagates in a specific medium, the velocity factor. For example, in RG-8/U cable, a common RF coaxial cable, the signal propagates about 3/4 the speed of light in vacuo. Or for light, the refractive index, n, expresses the ratio to speed in a medium vs. vacuum. In germanium, IR crawls at 1/4 the speed it would have in vacuo.