When I startup my system, start DOpus and open a folder with a number of subfolders containing video files for the first time, the folder-thumbs are generated slower then the second (and third, fourth, etc) time I open this same folder. This seems normal however, apparently the folder-thumbs are cached after the first loading.
But after a while the loading of video folder-thumbs slows down to it's initial slow speed and stays like that. So apparently, after a while the caching stops.
When I (full) exit DOpus and then start DOpus again, the loading is back to normal (1st time slower, 2nd, 3rd,... time fast).
Notes:
This happens only with video folderthumbs.
I have cache for thumbnails enabled.
Video thumbnails are not generated by 'movie' plugin.
If the Movie plugin isn't generating the thumbnails, Opus will be asking the Windows shell for them, and also won't be caching them itself (since the Windows shell caches them).
So behavior should be similar in both programs for that type of thumbnail. If it is the same, the problem is likely outside of Opus, and may be due to how Windows itself is/isn't caching things, or the involvement of 3rd party components involved in thumbnail generation (video codecs and similar, in this case).
Clearing the Windows thumbnail cache may help if the problem is there. My Clear Icon Cache tool should do that for you, or there are guides for doing it manually around the web. (Look for ones that talk about ThumbCache*.db rather than the icon cache files, which are probably not relevant in this case.)