ACDSee keeps a local database of everything. If I go through Windows Explorer it will take for ever. I have a folder for wallpapers with currently 2992 files in it, all anywhere from ~1920x1200 up to 5120x3200+ and about 7.1Gb total. Otherwise the only thing I can think of is finding some software that can render the thumbnails faster like ACDSee. Have you tried something like a web-based gallery like Koken or something else where uses can just download the files they want. The problem is that each new client is going to try to generate a thumbnail cache of their own, in my experience rewriting the thumbs.db file already present.
