A number of Windows iTunes users have reported extremely poor performance from the application. Symptoms include long lags when clicking on items, delays before playback starts, slow launches and more.
There are a few specific fixes that can provide a significant boost in iTunes for Windows speed.
Exclude iTunes library from antivirus software Launch your antivirus software package and exclude iTunes and the iTunes library (located in C:\Documents and Settings\yourusername\My Documents\My Music\iTunes) from the scanning routine. Because some iTunes library files are updated every several seconds, constant scanning by antivirus software can cause a massive slowdown.
Check for conflicts Use the instructions in this Apple Knowledge Base document to check for potential conflicts with iTunes. The suggested process involves using MSCONFIG to disable startup or system services that could be causing a slowdown, then re-enabling services one by one or in groups to check for recurrence of the conflict.
Add iTunes to firewall exceptions If you are experiencing particular slowness when accessing the iTunes store, try adding iTunes to the exception list in your Windows firewall. Instructions for doing so in Windows Vista are here, and instructions for Windows XP here.
You may also want to check any third-party firewall software and add iTunes to its exceptions list.
Turn off Genius It appears that that the iTunes Genius feature can cause slowness for some users. Try turning the feature off from the “Store” menu.
Update to the latest version Make sure you are using the latest version of iTunes, available from Apple’s Web site.