I could respond with more sarcasm, but instead, I'll just let this picture speak for itself:
/http://www.feldoncentral.com/forumpics/cpbackup_server_load2.png
The part where server load was 80-120? That's cPBackup running. For over 3 hours, websites on the server were extremely unresponsive, if they were accessible at all.
I'll give cPanel staff credit for being quick to respond, but it's often a link to the most obvious section of the cPanel documentation, even when a very specific question has been posed.
I'm running a cPanel / CENTOS 5 server on a quad core Xeon with 4GB RAM. Load is typically between 1-3, until cpbackup starts. Then it buries the needle for 3-4 hours. This will obviously preclude me from using cpbackup as long as it is not able to throttle back. I cannot have 4 hours of downtime every week.