Thumbnail mode is really inefficient and thus slow

[derf]

From TODO:
The thumbnail mode draws an image which has to be re-rendered as new
thumbnails are created, becoming quite slow for large filelists.
A workaround (--thumb-redraw) exists, but at some point a rewrite with a
GUI-toolkit or similar (or maybe writing a completely new tool for that and
removing the feature from feh) would be due.

[legrostdg]

Maybe something like https://docs.xfce.org/xfce/thunar/tumbler could help?