Zoneminder 1.32.3
OS's : Fedora and lubuntu
1 x BTTV camera
1 x Webcam
Using JPEG storage not H264
I've had a similar problem.
Cameras would use between 25 - 50% CPU load.
I replaced the GFX card and the BTTV camera drop to about 5 - 10% load that I'm used to seeing but the webcam has remained at around 25% load.