Then this may be caused by Qt itself which maybe tries to emulate a non-supported openGL feature for your card..
I discovered a clue to the nature of the resizing problem I have. Resizing
the application window and making it _smaller_ than the original size is no
problem. I can then resize back up to the original size without an impact
on th frame rate.
As soon as the window is re-sized to bigger than the original size, I get a
very poor frame rate.
This does recover when I resize back to the smaller size.