[ajosh]

The slowdown of fiber vs vacuum is about one third. A 33% reduction in latency is fairly significant for some applications. I've heard of financial trading firms using microwave relay towers for this reason.

The other issue is "long fat pipes." TCP is better tuned for it on machines now than it used to be but when you go over a long distance, the latency can decrease the usable bandwidth significantly. Of course, this seems to be more focused on last mile rather than back-haul. There are very few applications where a several nanosecond latency decrease matters.