Simple code like this may take a lot of time (1650 ms) to complete if QOpenGLWidget is in the same window and vsync is enabled (as opposed to ca 190ms without):

        for (int i = 0; i <= 1000; ++i)
            progressbar->setValue(i);

Perhaps QProgressBar::setValue could do update() instead of repaint() if it is on the same window with QOpenGLWidget, or have a flag that would choose which one to use.