Minor note---you can get much tighter theoretical minimum latencies than the ones listed in your table. 1. The table's mins divide straight line distance by the speed of light. This gives you the time it takes for light to travel in a straight line from, say, London to New York. However, your "real latencies" are roundtrip ("ping") latencies. Thus, you need to multiply all the theoretical latencies by two. 2. Data do…
The reason that light travels slower in fibre is because the refractive index of slica is about 1.5 while it is 1 in glass (in reality it's a bit more complicated, it's the group index that counts, which is also approx. 1.5 however).