PCMan looked promising but I need to filter directory contents. For example, geeqie and gthumb allow me to filter files using a REGEXP or a substring in a filename. Without that PCman is not useful to me. It was fast but that's not enough.
Second, I would like to show shortened/truncated filenames while viewing the thumbnails.Or even better, hide the filenames optionally.
Anyway, good luck with your work, will be glad to retry sometime later.