I don't deal with image/thumbnail files much, but this might help. At "Start, Control Panel, Folder Options, View (tab)", UN
check "Do not cache thumbnails" (i.e., DO
cache thumbnails). That might speed-up the display of thumbnails in all folders. You also have the ability to do this on a folder-by-folder basis, using "Tools, Folder Options..., View" in a particular folder.
The pop-up description of that checkbox option ("Do not cache thumbnails") is:
"Disables automatic storage of folder thumbnails in a cache file. When you store thumbnail images in a cache file, Windows can reuse them instead of creating new ones every time you open a folder. If you disable this automatic storage function, folders that contain thumbnails might take longer to open."
If this works, it may be that the first time you open such a folder, it will still be slow because Windows needs to create the thumbnail cache file initially. Subsequent openings of the same folder may be quicker.