The trick is to limit I/O bandwidth for rsync.

The –bwlimit option limit I/O bandwidth. You need to set bandwidth using KBytes per second. For example, limit I/O bandwidth to 10000KB/s (9.7MB/s), enter:

rsync –delete –numeric-ids –relative –delete-excluded –bwlimit=10000 /path/to/source /path/to/dest/?

10 megs a second is plenty of speed and will keep the server running smoothly when your grabbing allot of files off of it.