Light in a glass fibre travels at about two thirds of c

The refractive index of the glass in a communications fibre is about 1.5, so a signal in the fibre moves at about two thirds of the speed of light in vacuum.

Vacuum light covers about 300,000 kilometres in a second. In silica it covers about 200,000. A path of 6,000 kilometres of fibre — a plausible transatlantic route, with the extra length of not being a straight line — takes about 30 milliseconds, one way, before any equipment adds a microsecond. A round trip is twice that, and the route you actually get may be longer than the route you wanted. Traders have paid to lay straighter cables because, at this speed, a shorter path is a earlier arrival, and earlier is the whole product.

Hollow-core fibres, still mostly in the laboratory and in short specialist runs, let the light travel in air inside the glass and so run faster. They are not the cables under the ocean. The ocean is silica, and silica has an index.