I've never known a Windows user who didn't loathe iTunes, so I can only conclude that it does Very Bad Things on a PC.
I'm getting a little tired of it myself. It no longer seems like a particularly well-thought-out solution for media playing.
There's always a first then right?
Seriously, I don't know why you guys say it's all slow on Windows.
I had a library of about 15GB of just music (I know, it's minuscule compared to some of yours) and on a 1.6Ghz Pentium Dual Core and 1GB RAM it ran fine. It opened with the same speed that it does on my Mac, and I saw no problem with it.