Signal speed in fiber is about ⅔ of that in vacuum (https://en.wikipedia.org/wiki/Optical_fiber#Refractive_index https://en.wikipedia.org/wiki/Optical_fiber#Ref

Signal speed in fiber is about ⅔ of that in vacuum (https://en.wikipedia.org/wiki/Optical_fiber#Refractive_index https://en.wikipedia.org/wiki/Optical_fiber#Refractive_index), but fiber won’t be straight-line between sender and receiver, light doesn’t move in a straight line inside the fiber, and the switched adds delays. https://www.pingdom.com/blog/theoretical-vs-real-world-speed-limit-of-ping/ https://www.pingdom.com/blog/theoretical-vs-real-world-speed...: “you should probably double the “ideal” response times shown above for a more realistic target to aim at“ So yes, ⅓ of light speed in vacuum seems a decent heuristic.