Actually, I'm a bit surprised that the card works with iCamSource at all. A lot of video capture cards aren't recognized by the OS as a webcam.
I'm guessing that the the high processor usage is probably due to the raw image size from the video card or the framerate from the video card. I know that some Firewire cameras will use more CPU because they have a higher framerate than most webcams.
Does the card have any settings where you can set resolution, compression, or framerate?