What is the difference is velocity, comparing signal on copper and light in “glass”? My impression is these velocities are not much different, both being fairly close to “c”.
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.