The speed of light through fiber is about 1/3rd slower than air, this makes the biggest impact to latency over distance. This is why microwave radio is sometime

The speed of light through fiber is about 1/3rd slower than air, this makes the biggest impact to latency over distance. This is why microwave radio is sometimes used where speed is more essential than bandwidth, for example companies have established microwave radio links between Chicago and New York for trading for this reason. New advances such as hollow core fiber might change this equation, because from a simple perspective, they essentially act like hollow wave guides that AT&T long lines was once building to carry microwave radio in tubes across the country (making them impervious to atmospheric issues, etc). I don't know too much about the practicality of using this kind of technology yet over global distances. Another thing to watch for of course is SpaceX's plans to build a constellation of 4000+ communication satellites. They will be using "free space" like optics. Essentially in space you know where everything is, you don't need to use radio, you can point lasers between all your satellites and use tracking to keep them aligned. This provides huge bandwidth for them when you consider all the multiple paths between the 4000+ satellites. Plus because its traveling at the speed of light it will be ~50% faster than packets traveling through submarine cables. This might end up being the winning solution for reducing latency in global distances IMO.