Why is GDM faster than a browser?

A browser fetches a file over one connection. GDM asks the server for several parts of the file at once, up to 32 connections (128 if you allow it), and keeps re-splitting: when a connection finishes its part, it takes over half of the biggest part still left, so no slow connection holds up the end. It also remembers servers that refuse extra connections and stops asking them. See Connections and speed.