Apple - Using FaceTime without the video

Yes — if you simply minimize the FaceTime window (⌘ Command+M) or hide FaceTime (⌘ Command+H), video will pause while audio still functions as usual.

(Similarly, on iOS, pressing the home button will pause video and allow you to use other applications while audio still works.)

All reports seem to indicate that there is no way to use facetime on the mac without video. You can continue a facetime call audio-only on iOS devices by pressing the home button, but oddly enough you can't disable video for the mac.

One suggestion I read awhile ago for low end machine was to place a piece of black paper over the camera. It takes much less processing power to encode a static image of mostly black than your face.

Minimizing the window will disable the video reception decoding, and may save you some processing power. Taping over your camera and minimizing the window might give you enough processing power back to use audio comfortably.

You might also consider skype or google voice if all you need is audio.