The speed a web site can transfer at depends on a lot of things. One of them is how much traffic it is handling. So if a lot of people are using that web site, it is going to slow it down. The speed of the line to the web site and how many others are sharing that line is a factor. The computer and its disk that are used for the server can affect the throughput. Caching and buffering can help speed things up, but that is something they probably have down fairly well if they serve a lot of people. You want max speed? Do your transfer off hours when the network and computers are not busy.
There can be other factors, but those are the major ones.