When you're converting song formats in iTunes, it can take up to 5 seconds per song. That's pretty quick. But if you have several thousand songs in your iTunes library, it's going to take many hours for the conversion of your entire library. There is a trick you can use to speed up the conversion so that it's nearly 10 times as fast. Once the conversion starts, go open up the "Preference" in iTunes. That's it. Just let the Preferences window open while the conversion is taking place and it'll happen 10 times faster.

reference: http://www.macosxhints.com/article.php?story=20090218080208390