Sluggish response in iTunes often relates to HDD condition, capacity, how full it is, fragmentation, etc. It's pretty well known that as you fill your HDD past 50% the performance gets progressively worse. At 90% it's pretty much useless. However, HDDs are cheap, you could get a "media drive", and use it only for your music, move your library, and clear some space on the main HDD, then defrag. Your media drive could be internal or external. If external, and your PC has USB3, go for a drive that does that too. Even if you had an older drive laying around that you could reformat and use, that might be an improvement.
Another performance hit sometimes comes from a disorganized iTune library. Sometimes media and data gets spread around the computer in different locations. If the library has some years and version updates on it, you may want to recreate it on your new HDD using "consolidate". You can do this through iTunes, and build a new copy of the library in one location. The instructions on this don't belong in this thread, you can find articles via google.
RAM is now pretty cheap too. I just bumped my laptop's 4 gig to 16gig. It's like a completely new computer now, the performance bump was shocking. But iTunes always ran fine before, so I'd probably do the HDD upgrade and library move first, memory last.
Slow syncs are related to how much iTunes has to sync and the sync method. If it's a direct connection, that's the fastest, and we're all using USB2 now, right? You can actually sync an i-device using USB1, but it will take all day. AirSync is convenient but also slower if your device uses that. If you have an iDevice with photo capability, syncing photos will slow you way down. Photos resync all the time, so the fewer you sync the faster. Just import all you have to the PC, organize the ones you really need on the device into a folder, clear all photos off your device then resync only the photos your really need to have. Once you've rebuilt your library you may also see a sync improvement.
