I know that I read somewhere on this site of someone else complaining about slow performance and someting about using a Thunderbird to Gigabit Ethernet Adapter to improve performance.
A Gigabit Ethernet adapter might help if you were accessing files on a shared file server in your house. (802.11 'n' WiFi aims for an effective speed of 100 Mbps, or about 1/10th the speed of Gigabit Ethernet.)
However, in this case, the bottlenecks are likely (a) your Internet connection, or (b) Apple's servers. Unless you're paying hundreds or thousands of dollars a month for Internet service, you are probably getting access speeds of 0.8 to 50 Mbps (i.e., within the capacity of 802.11 'n').