I can currently see PL creating thumbnails for 21 minutes now with a filmstrip showing 8.5 images. The folder size for the previews hasn’t changed for a while. But it looks like PL is happily creating thumbnails even with an otherwise untouched PL. When I select a different image, the size of the preview folder increases considerably.
Creating thumbnails is done at about 160% CPU. Quitting after about 25 minutes shows that the cache lists 1650 thumbnails (the number of images in the folder) and 16 previews (about 2 film strips worth, I scrolled once to select a different image) File type is “opentry” which can be opened with “Graphic Converter”.
All of the above is on macOS. On Win YMMV.