Ah. I use capitals like I emphasize speech. "Here we have a door. The room is empty. But behind *THIS* door, ..."
What you're doing is just using a network share. What confused me is that the My Book Live has a built-in iTunes server, and I assumed that's what you were talking about.
Hmmm. I've just always had MY iTunes stuff local; I've never tried sourcing it from another location for the purposes of Syncing.
My wife uses the same music library, but from her laptop. So, she's connecting to it via network.. I haven't noticed it running any slower, but she syncs different stuff than me.
So, your laptop, is it WIRED or WIRELESS? Wireless can cause all kinds of performance issues; just because it says it's 54 megabit per second "G", doesn't mean you'll get anything close to that. Especially if you have more than just a few devices using wireless.
A basic way to measure speed on the network is to copy a file across, and just measure the time it takes using a stopwatch. Find the biggest, fattest file you have, and copy it over to the MyBook. Just measure the time it takes. Divide the size of the file (in BYTES) by the number of seconds, and that will give you Bytes per Second. It should be CLOSE to 10 megabytes per second, or faster.