What is Latency floor, group index?

The latency floor between two sites is the one-way time light needs along the geodesic between them, $d/c_0$ in vacuum, with $c_0 = 299\,792\,458\,\mathrm{m}/\mathrm{s}$; in a medium it is $n_g d / c_0$. The group index $n_g$ of a medium is the ratio of $c_0$ to the speed at which a pulse, and so information, travels through it: about 1.462 in standard single-mode fibre at $1550\,\mathrm{n}\mathrm{m}$, about 1.0003 in air near the ground.