In a fiber-optic cable, they slow down closer to 122,000 miles per second. The loss of speed measures roughly 8. 2 microseconds per mile, or 0. The fiber latency calculator helps determine the time it takes for data to travel through a fiber optic cable between two points. In fiber optics, the. Signal speed is entered as a fraction of the speed of light, so a value of 0. The calculator then returns repeaters required, propagation delay, total one-way latency. Measured in milliseconds per mile, latency defines the time that elapses between the moment you send data across a network and when the information reaches its destination. 5 µs/km, copper cable sends it at ~5.