Before Catalina 10.15.3, Time Machine seemed to work well, at least when backing up to local storage. For years, I’d kept my backups on a Promise Pegasus RAID with its four spinning hard disks. When I eventually replaced them with an OWC Thunderbay 4 full of costly SSDs, those backups got pleasantly quicker, at least until I updated my iMac Pro to Catalina 10.15.3 on 28 January 2020.
Something broke then, and full Time Machine backups suddenly took an age. I discovered the offender after looking in the log: the hidden .DocumentRevisions-V100 folder, containing the macOS versioning system database, was bringing each backup to a standstill, sometimes only copying one tiny item a second. I reported this bug, but it wasn’t fixed in Catalina, not until Apple released Big Sur, by which time Time Machine was already backing up to APFS volumes.
The fix adopted by Apple was essentially the same as I had already proposed and implemented myself: add .DocumentRevisions-V100 folders to Time Machine’s exclusion list. Since then, others have encountered similar problems elsewhere. It has been noticed by many in their Photos libraries, in Apple’s Xcode app, and in strange out-of-the-way folders. The only common factor is that, when trying to back up folders containing seriously large numbers of very small files, some of which may be hard links, the rate of copying falls to ridiculously low numbers.
Read more at eclecticlight.co