Thanks Nudel but I've already seen [Crash, exit or high CPU when viewing certain directories) I've tried turning off all viewer plugins to no affect.
I think my question is a little misleading in so far as it really applies to the caching of the thumbnails. I should make it clear that if the thumbnails are cached the images do load quicker but slower than it used to be. It's during the caching process that the CPU becomes overloaded my PC slows to a crawl