Those are already selected, and the thumbnails have been long-since generated for most of my gallery content.
Let me give an example. Let's say I have 4 albums, album1, album2, album3, album4. I go to my gallery main page which loads quickly. I then pick album1 and I see all of my album1 sub galleries and images displayed. If I select "edit album" at this point from the admin toolbox, I can go out for coffee (and maybe lunch) before the album edit page is displayed.
It will take upwards of 5 minutes to display the edit album page. If I look at the system I'll see the php-fpm process in top. If i strace that process I see it calling stat() for every file in every album, not just album1 and its sub albums. It will call stat() on every image in album1, then every image in album2, then album3 and so on. Since it's being called from php stat() won't tell me where this is being done, but it really looks like a bug that the system is trying to inspect every single image in the entire gallery whenever I edit any album.