Fixing Slow TimeMachine Backups
fs_usage backupd
and look for the THROTTLED entries. If you see them, the backup is throttled.
So if you have a ton of files, just the time it takes to do the I/O takes forever, even if the files are small (because it performs a bunch more I/O operations around xattrs etc. than it used to).
sudo sysctl debug.lowpri_throttle_enabled=0
It's also a good idea to re-enable the throttling after your backup finished successfully with the following command
sudo sysctl debug.lowpri_throttle_enabled=1