It's noticeably slower at loading than other software, like Microsoft Office or Adobe Creative Suite - that's loading it from a SSD. It's also less responsive than most other applications, including web browsers. It's not that it has really bad performance; it's just that it's far from good. It's been chronically underdeveloped and when Apple has added features they have tended to be pretty disastrous, like the Ping and Genius (got a lovely "An unknown error occurred (-3212)" warning just a moment ago).
Apple needs to properly optimise it for Windows and ditch the horrible, proprietary interface. Hopefully iTunes 11 will improve things but I'm not overly optimistic based on Apple's track record.
I do hope that we'll see a "native" interface with iTunes 11 for Windows, but I don't think it'll ever happen.
I haven't experienced any performance issues in a few versions though. iTunes 10 has worked for me pretty well, and I have several hundred GBs in my library. Admittedly, I only use it as a music player, nothing else, so I don't know if that affects it or not.
Now... release iTunes 11 already!!