network – Why is a Time Machine backup so slow?

Despite having a gigabit hardwired ethernet connection to my Macbook Air, when I have Time Machine backing up to an iMac also hardwired on the same switch in the same network, the transfer speed is often less then 1Mbit per second. If/when I copy a file from the Air to my iMac, I get a speed of about 65Mbit per second. As it is, it takes about a month to get the 1Tb disk in my Air backed up, all that time sitting there with a wired connection and doing little else. If I could get 65Mbit/sec with Time Machine, it would take less than 5 hours to backup that 1Tb disk. How can I get the initial backup to go as fast as possible?