QAbstractVideoSurface latency problem

[realrunner]

I'm using a QAbstractVideoSurface to grab video frames from a webcam. It works great but for one problem.
It will only present frames (Qt framework calls the present method) at 12 fps. I've gone as far as hardcoding (as a test) the FPS setting directly in the AVFramework driver (on mac, in the qtmultimedia plugin code) to 30. I know my webcam supports 30.

Does anyone know if this latency is expected and if there is any way to get closer to 30 fps?