Create sub Folders and categorize the jpgs.
The Thumbnail display operates as a background task and may be timing out due to the number it is trying to display while other tasks are running on your computer.
By subdividing you will probably get a reasonable speed of display, if using subFolders is not practical then it is a matter of waiting and refreshing until the background task has completed fully.
More a follow-up than an improvement. I was going to follow through on your idea, but when I got back together with the user, and we went to the directory in question, all the files had thumbnails. Don’t know why, and it seems to be still working.