I wonder if you got the new version of iTunes. Mine runs like a pig while its doing all the album imports and setting the gapless playback information. In the window at the top of iTunes, it'll say something like "Determining Gapless Playback" when you first start it. Hit the X at the right hand side of the window and your machine should speed up a bit.

Pisses me off no end as I can't see an option to turn it off.

Cheers

Matt