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.
Propagation in wire can be as low as 3.7m/s.
Not a typo.