It only requires CPU usage when it's streaming the contents of your screen. For Youtube/Netflix, all it does is tell the key the url of the stream and everything is done either on the key or the server.
If anyone has this device, what is CPU usage on the source like when tab-casting?
And now people have published the extension of that, which is that you can cast whole screens and arbitrary content just as video streams - http://www.droid-life.com/2013/07/26/tip-chromecast-can-cast...