Why does a backup take so long?

The first backup of a large library can take days. Parachute Backup saves every photo and video individually through Apple’s Photos framework, so capture dates, locations, Live Photos and edits are preserved, and tens of thousands of items take much longer than copying a few large files. Items that are only in iCloud also have to be downloaded first.

Later backups only transfer what is new or changed, but they still check every item (why?). That check is fast on a directly connected drive and slower on a network drive or in the cloud.

To speed things up, use a directly connected SSD, narrow the date range in the Photos settings, or turn off Shared Albums if you do not need them.