How are you gauging the speed? Can you tell its running at fast ethernet somehow? I have heard these are really slow due to the CPU rather than the network. Having said that, Gbit ethernet should be auto-negotiated requiring you to do nothing, so back to my first question, how do you know its not Gbit ethernet?
I would check to see if your router firmware is current. Also, check to see if there are updates for the WD mybook.