Be ready for some albums to take AGES to download huge booklets in large 20MB PNG files. Watch for the progress as the colour changes on the Picard Icon in the toolbar, and filenames flick through at the bottom left of the status bar. Well worth the wait though.
If you are just wanting to update tags and not images, temporarily turn of the Cover Art Archive from the providers. This is the trick I do if I previously downloaded all the artwork and today I just need a quick tag update