I would recommend you install the systat rpm and then use sar and iostat commands to see if you can identify where the bottleneck is occurring.
I installed sysstat and played with it a little - wasnt getting anywhere. I did some looking last night and came across a kernel tweak echo 10 > /proc/sys/vm/swappiness
This has help the responsiveness during the rsync to the external USB drive which was the worst case.
Thanks,
Jerry