Do you have more than 10,000 songs in your collection?
These days music collections are usually a couple of thousand songs big. For these kinds of libraries, beaTunes features something called automatic synchronization. Whenever you switch from beaTunes to another application and back, beaTunes will try to make sure that changes to your music files are automatically imported. This is very convenient and no big deal—unless of course you have a very large collection. And with very large I mean ten thousands of songs.
Synchronizing external changes with the built-in internal database can take a while, especially when your music resides on a slow external drive, e.g. a NAS. You'll notice that beaTunes becomes sluggish, you may see timeouts, and in its status bar beaTunes displays something about synchronizing. Fortunately, this annoying behavior can easily be avoided.
Simply open the general preferences and turn off Synchronize on window activation. Of course that means that external changes aren't automatically discovered by beaTunes anymore. But this issue can easily be solved as well. Whenever you believe, beaTunes has gotten out of sync with your files, simply run
Synchronize. This lets you synchronize your metadata at a time of your choosing, and only when it's really necessary.