How does it act if you only run the iCamSource for the IP camera? If the CPU is struggling with two instances of iCamSource, it may behave better with one. If you can run that one instance successfully, it might indicate that your guess is correct.
We have seen cases where IP cameras require high CPU usage because they are outputting high resolution MJPEG data. Higher resolution MJPEG data just means more image processing is required for resizing, compression, motion detection, etc. Check your IP camera settings, try to find out what resolution it's outputting. Try to set it to 640x480 or lower (if possible).