Time Machine in the latest version of macOS has problems making and restoring from backups
Time Machine, the official macOS backup utility, is having major problems creating and restoring from backups in the operating system's latest release. The issue can be as severe as not being able to complete the first Time Machine backup being made for a new volume.
Dr. Howard Oakley, curator of The Eclectic Light Company blog, has put together a detailed post describing the issues that Apple's official Time Machine backup solution has in version 10.15.3 of macOS. The problem seems to lie within the first backup made via Time Machine on 10.15.3. The average writing speed for that first backup up was varying between 0.0-1.29 megabytes a second. This means that a full first time backup via time machine on Oakley's test machine took 14:22:36.
Oakley compared Time Machine to Carbon Copy Cloner and ChronoSync in terms of read speed and backup completion times using the same solid state drive through the same iMac Pro and Thunderbolt 3 enclosure. Both programs performed "far superior" in comparison. According to Oakley, this may be because of the "large Revisions/Versions database stored on the volume being backed up, in the locked and inaccessible folder .DocumentRevisions-V100 at the root of the volume." Excluding this from a Time Machine backup, however, will mean that you lose previously saved versions of documents on the volume being backed up.
Apple has been informed of the issue. In the meantime, if you are running into extremely slow or non-completing downloads, Dr. Oakley recommends the following procedure:
"Cancel [the Time Machine backup], remove the incomplete backup, add the .DocumentRevisions-V100 folder at the root of each volume to be backed up to Time Machine’s exclude list (press Command-Shift-. to see hidden items in the file selector dialog), then try again."
The Eclectic Light Company