Light can travel in air much faster it can travel in glass.
Similarly, copper is faster than glass too (but not faster than air). The trade folks refer to copper in terms of "velocity factor". Example, some arbitrary RG-6 I'm looking at has a velocity factor of 81%. Equivalent refractive index is 1/.81 for ~ 1.23.
Refraction index of light in fiber is ~1.44. Rule of thumb for packets in transit is 200km/ms.