Why In free space electromagnetic waves travel at c, about 3 x 10^8 m/s, but in any material they slow to v = c/n where n is the index of refraction. That index comes from the medium's permittivity and permeability (n = sqrt of the relative values multiplied together), so the electrical properties of the material set the propagation speed. This is the same quantity that governs velocity factor in feed lines and bending of waves in the ionosphere.
Watch out Resistance and reactance describe circuit behavior, not wave speed in a medium; birefringence is a material having two different indices for different polarizations, and evanescence describes a non-propagating field that decays with distance.
v = c/n: the index of refraction is literally the ratio telling how much a medium slows a wave.