modelmaker,
MC uses its own fast thumbnails in thumbnail views. Accessing the source image files directly would be too slow. If you have a thumbnail for each cover art then the high CPU usage is caused by something else.
To Modelmaker,
However, you may just "think" you have all your thumbnails build but, reading your posts, I would gess not unless you have already scrolled down ALL your files at least once since you installed MC.12 and with your number of files it may be unlikely.
If I'm not wrong, there have been at least two major changes to the thumbnail engine since the introduction of MC.12 that required building new thumbnails.
You can try what Matt suggested:
Go to Tools -> Options -> Tree & View -> Thumbnails -> Build missing thumbnails
(Let it run overnight)
Then you positively will have all your thumbnails build.
Try to scroll again your huge playlist and see if the problem persists.