How to get realtime from ofxQtVideoSaver


I’m trying to capture the video output from my application.
I’ve tried many addons without success.
So…I’m using Screenflow but I wonder why I can’t manage it in OF.
My main problem is the size of the capture.
for example with ofxQtVideoSaver. I’ve started with the ofxQTSaver_simple.
I’ve had this line to the setup

#ifdef CAM_HD  
	camWidth 		= 1280;	// try to grab at this size.   
	camHeight 		= 720;  
    camWidth 		= 320;	// else this size   
    camHeight 		= 240;  

If I have NOT defined CAM_HD in the header, the capture is realtime.
But if I have defined CAM_HD int the header, the capture is faster than realtime.

Please find attached the project

Any guesses ?