Weird framerate drop when running openni without screen

For an installation I am running an app on a mac mini without a screen. For tracking users i use ofxOpenNi.
Everything works fine just until ofxOpenNi detects a user. Then everything just gets terribly slow.

If I attach a screen or connect to the mac mini through screen-sharing, the app speeds up to normal speed again until I disconnect.

If I set vertical sync to false, things seem to be a bit better. I only have to connect once to the mac mini to get rid of the framedrop and if I disconnect again, the app stays at its normal speed.
But since the application should start up automatically, this would be no option.

If I start my app in fullscreen mode, every thing seems to be normal.

Can anyone point me in the right direction?

I have ran into this when I had an OpenNI based app that ran slow when minimized.

My guess is that there are draw commands using OpenGL somewhere that aren’t being executed or executed slowly.

some possibly related stuff here:

Thank you.
I solved this for now by connecting a old screen to the mac mini. Without even plugging it to power.
Everything seems to work fine but it feels a bit like voodoo…