There's also hollow core fiber, which is pretty close to speed of light in a vacuum. 2.0e8 m/s for fiber, 2.3e8 m/s for copper, pretty close to the full 3e8 m/s for hollow core.

No glass, just some reflective coating on the inside of a waveguide (hollow tube).

https://azure.microsoft.com/en-us/blog/how-hollow-core-fiber...