If you have a large iTunes collection with a lot of Smart Playlists set to auto-update, you may experience very slow performance when using iTunes' ability to convert a song from one format to another (using the Advanced -> Convert to... menu item). You may not notice the slow down when converting just one song, but if you're converting hundreds or thousands of tracks, the process can get very, very slow. The cause of the slow down is apparently the combination of the large library size and the auto-updating Smart Playlists. As each file is converted, all of the Smart Playlists are updated, and that can be quite time consuming.
There are a couple of different workarounds for this. The first is to use an external program to convert the tracks, and then import them into iTunes. I know of a couple such programs--X Lossless Decoder and Max--but undoubtedly, there are many more. To use an external program, you'd export your tracks (non-FairPlay-protected) from iTunes, convert them, then import them back in. In the end, though, this might be as time-consuming as converting them within iTunes.
The other workaround is a bit strange, but it seems to work--while I haven't tested this one myself, Macworld contributor Kirk McElhearn did, and he's told me it works as described. So what is this workaround? Go ahead and start the massive conversion within iTunes, but just after it starts, open iTunes' Preferences window.
iTunes' Preferences window is a modal window, meaning that it traps all requests to interact with the program--whether you make those requests (notice that you can't change the song while the Preferences window is open), or the program itself makes those requests. In this case, it's that second behavior that does the trick--because the Preferences window is onscreen, iTunes can't update all the Smart Playlists while it's doing the file conversion.
Let the file conversion process complete, and then close the Preferences window. iTunes will then update everything in the library, during which time you may not be able to use iTunes itself. However, Kirk reports that this is still notably less total time than if you were to let the updates happen during the conversion. In essence, you'll only have to wait for one "big" update, instead of many hundreds or thousands of individual updates.
This story, "Save Time When Converting ITunes Tracks" was originally published by Macworld.