Slows down at a certain point on new pics, possibly related to metasave filesize (modded or vanilla, seems to chug around 29-33kB, which is around the 4th to 6th character in vanilla).
I don't know why this file is encrypted in the first place, but the incredibly obvious and otherwise not needing to be explicitly stated impacts of that aside, an easy optimization would be to only update this file on day transition/quit to main, that way it's not not pausing/interupting to autosave on every single new pic, which could easily be 15+ times in vanilla, and 50+ in modded cases.
(I'm worried how images are referrenced in the save structure, but hopefully images are assigned a computationally simplified ID as they are loaded and the simplified ID is then stored in the save, and not something like, storing the filepath/name.extension/string of every single image within the save data...)