Nick Hilliard writes:

The other problem with it is that if you've got lots of files, it tends to consume vast amounts of memory. Last time I used it for syncing a large volume (over 3M files), it required about 600 megs of memory (for which swap works fine).

Yes, that's true. It can also get a bit CPU bound. That's why it can pay to break up the rsync into multiple processes (one for each subdir) and split them across multiple processors.