I had a quick nose around for you, but I couldn't quickly find a related tweak to suit your needs, I'll have another look later. However to disable a folder from displaying thumbnail previews of images and videos when you open a folder, simply click on 'view' and change the view selection to anything else other than 'thumbnail'. However this method is hardly super effiecient.

You could also use TweakUI (the microsoft power tool, see the tweaks and tricks thread) to lower the thumbnail quality so the time it takes to load the previews is shortened.