Very slow photo library backup on Mac

[Philipp]

Hello!
I use FreeFileSync to back up my local photo library on my Mac. It used to work well and usually took about 5 to 10 minutes. For several days now, I’ve had an issue where the backup starts normally but then slows down drastically. FreeFileSync reports that it would take around 550 days to complete the backup.
Could this be a bug? I have no idea what might be causing the problem.
Thanks for any help!