Here is something odd. I am currently subscribed to twenty six podcasts and if I open up iTunes and hit the Update button iTunes.exe starts using up all my CPU cycles, it takes about six minuets to check all the feeds and some of them fail to update (just done it now and fourteen of them have failed).
If however I press Ctrl+Alt+Delete to get into the Task Manager, right click on iTunes.exe and set it's priority to low. Not only does it do the whole podcast updating thing in twenty seconds, but they all successfully update.
I don't know why it acts like this, but it's odd and needs fixing.