This isn't the only performance problem with rsync. I have a filesystem full of home directories. A 600G filesystem with about 500G filled. Doing an rsync between it and a backup server takes over 24 hours even though less than a GB typically changes between backups. Worse it consumes 1.6GB of ram and unless reniced makes the performance of the file server stink as it becomes entirely disk bound with a fair amount of CPU as well.