It’s actually much more complicated, and much more varied, than we expect.
“For example, in copper at 5 GHz the skin depth is about 1 micron, the wavelength is about 6 microns and the velocity is about 30,000 m/s.”
4 orders of magnitude at the frequencies we’re considering.
Great info. Thank you! I knew skin effect (Litz wire), but never associated it with that much loss of velocity (in or on copper).
Unrelated odd effect is "photon velocity" inside a star. It takes a long time for a photon at the core to make it to the surface on account of frequent absorptions and re-emissions.