Too much latency between user input and the corresponding change in graphics display

To reproduce:

Open the QtQuick Controls gallery, and open the "Progress" tab.
Move the slider handle back and forth. You will see that the slider handle is lagging a lot behind the mouse cursor when moving the mouse cursor.

Compare it with QSlider (launch the examples\widgets\widgets\styles example for instance), and you will see that the QWidget-based slider has much less latency between the user input and the graphics display.