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