seems to be more related to "otherly media", movies, sounds, pdfs etc, even if not selected in the options.
EDIT : now that i've scrutinized it a little more, changed some options (file list check boxes) seems it's the sub folders messing it up the most, and specifically on the "otherly media" (non images), BUT also with just plain images,
sub folders seem to be the issue ... possibly the recurse method, maybe it's going deeper than just the 1 level down to thumb the quad images
perhaps this will be beneficial, with plain images (bmps/jpgs) a sub folder with say 5 images will take sometimes 2-5 seconds to process alone (showing an icon, cached or not.) mind you the top level is not a folder with a ton of sub folders, i think it's a half dozen at most. system is winxp64, AMDx2 6000 (~3Ghz each core) 4gig ram. i also use acdsee browser(2.41), on caching images both applications are very fast, almost instantaneous even on large folders. but to focus this bug report more i'd say it's in the folder subroutines.
kick @$$ software tho, love the movie/sound built in player, been using xnview more than windows explorer. also love that it only adds to document history when you spawn to external application, cause sometimes i'm searching for an image for my son's web page and my recent documents gets flooded with my trial and error searching.
hope the info helps
keep up the awesome work
PS ... in the screenshots i've seen the thumbnail view have a folder icon with ".." for folder up.... i've searched the help file and kicked around the forums, but haven't tracked down the exact option to show that, is that still a feature?
The browser has a severe slow down and sometimes causes it to not respond and fail, also during it's "caching" or whatever it's doing with the status bar progress indicator, if you go to options, change folders, try to close the app or do anything .. it halts the program. 
