Apple Support Discussions forum user "whato39" reports:
Just updated to the new itunes version, but when it begins to update itunes library, it's taking a **** of a long time to complete, so far the computer has been on for 26 hours straight and it's only half way through, this has never happened before in previous upgrades. Any ideas anyone, I have backed up the library and done a clean install of itunes, but when i drop the library details back in the right place, it goes through again updating the library and taking forever.After 90+ hours, one user, "Dan Tortorici", discovered his hangs were due to a movie file that was having trouble updating. His solution:
1. With iTunes running (or hanging you may prefer)
2. Open the Activity Monitor app that is in Utilities
3. In the Process Name column find the iTunes process and select it
4. Go to the top of the window and click the blue "Inspect" button
5. There are 3 tabs on the window that comes up, select the Open Files and Ports tab
6. Scroll down to the bottom and look for the last file it opened - this is the culprit
7. Force Quit iTunes
9. Goto the location of the 'culprit' file and delete it.
10. Run iTunes once again.
If the hangs persist, users may have to complete this process again, looking for more files causing the issue. Users that store their iTunes Libraries on an external drive may try the process described by ASD user "ATay71". First, disconnect your external drive from your Mac. Open iTunes and let the library update--at this point it should not take long. Reconnect your external drive (your songs may appear as missing at first). Shut down and reopen iTunes. Everything should be back to normal with your library updated.