How old is your computer? Older computer = less processing power.
How many cameras are you using? More cameras = more processing power used.
What type of cameras (USB or IP cameras)? USB cameras use more processing power, as they provide a higher frame rate, etc.
What is the resolution of the cameras? Anything over 640x480 will use more processing power, as they need to be downsized to 640x480 for iCam. If possible, change the resolution to 640x480 for the cameras. You can also try reducing the image quality in icamsource, as well as decrease the sensitivity of motion detection.
What is the frame rate of the cameras? Higher frame rate = more processing power used. If possible, lower the frame rates of the cameras to 5 fps.
Do you have audio enabled? Audio will use more processing power. Try disabling audio on all camera feeds.
With all of that being said, this is what I'm experiencing...
My system: 4 or 5 year old dual-core processor, 4 IP cameras (one with audio enabled), all cameras set to 640x480 and 5fps, motion detection/recording/notification enabled on all cameras.
Processor used when cameras not being viewed via iCam app: 12-19%
Processor used when viewing all cameras (thumbnails): 22-33%
Processor used when viewing one camera feed (with audio): 20-30%
I'd probably expect USB cameras to use roughly double the processing power...maybe even more, especially with audio...and especially if you're using a higher-resolution webcam (which are very common these days).