If you direct connect the MyBook Live to your PC I believe you need a crossed over network cable and the MyBook Live to have a static IP as well as your PC having a static IP address on the same sub-net.
The normal connection is that there is a router between your computer and MyBook Live and the router sells your PC and MyBook Live what IP addresses they are to use.
There are a few issues what can affect data throughput. Maybe the router’s processor can’t cope? Copying over a lot of small files will have a performance impact as the MyBook Live and PC have a lot of house-keeping to do between files being copied.
Now, if you have large files then there is a lot more simple copying of data more of the time and then the file-system updates and stuff in-between.