[myself248]

Velocity factor in most cables is between 0.6 and 0.8 of what it is in a vacuum. Depends on the dielectric material and cable construction.

This is why point-to-point microwave links took over the HFT market -- they're covering miles with free space, not fiber.

[jcims]

I always thought it was about reduced path length. Interesting.

[cycomanic]

It's both. Those links try to minimise deviation from the straight link (and invest significant money to get antenna locations to do that), but they also use copper/coax cables for connecting radios as well as hollow core fibre for other connections to the modems.