Part 1.
I rebuilt my thumbnails. It took over 30 minutes.
I had "erase orphans on exit" enabled and after restarting MC I had this:
21186 thumbnails (size on disk 164 MB)
My library consists of:
Total amount of library items: 30689
Audio: 26182 - 16563 music files with cover art linked, 9619 without cover art
Video: 275 - 1 video file with cover art linked, 274 video files without cover art
Images: 4088 files
Documents: 150 files
Internet radio station links: 4 addresses
~nodup=[Image File] returns 1138 files (in all media view)
16536 + 275 + 4088 = 20719 (should I have this amount of thumbnails instead of 21186?)
How does the thumbnail math work?
Anyway, it would be nice if MC could avoid making duplicate thumbnails for each album track, but I guess it's easier (and faster?) to build a new thumbnail for each item instead of accessing the thumbnails with internal linking.
My logic would say like this: 1 cover art image file = 1 thumbnail, and that single thumbnail would be used for all album tracks. Images inside tags, video thumbnails and actual images in the library understandably need a thumbnail for each library item.
Using my logic I would have 1138 + 274 + 4088 = 5500 thumbnails
Part 2.
Now I have only 24 thumbnails, because I closed MC when another library was loaded. That library has 24 items.
Surely, it's not going to make using MC faster if the thumbnails need to be constantly rebuilt.
Part 3. (Edit)
Also, MC loses all previous thumbnails if the libraries are loaded back and forth without closing MC.
I found a bug too: The Tiles list style does not show images.