Light sets up the maximum available speed with near 300.000 kilometers each second, so electricity must be below this threshold. There are who think that electricity is simply another propagation of electromagnetic energy, so the distinction doesn't really apply, only accounting with the permittivity and permeability of the medium, slowing down electricity, and its signal integrity.
Electricity for sure employs different mediums to propagate due to inherent conduction, induction, field lines or radiation between other factors. Each medium has different permittivity and permeability constants, thus knowing them helps in the task of knowing the speed.
The complex and amazing world of electromagnetic emissions is right now a point of concert in multiple industries due to the necessity of passing certain test to meet EMC (Electromagnetic Compatibility) in developed countries. Normally the regulation appears less tolerant toward residential scenarios than in industrial ones.
Any conductor could radiate energy at an incredible speed if electricity goes there. The so-called improvised antennas. This is a world full of misconceptions and improvisations when it comes the design of printed circuit boards. However, electricity is still not known at its fullest, and still in investigation so it's pretty reasonable.