Use the attached test application to reproduce the issue.

aboutToBlock signal from QAbstractEventDispatcher is used for triggering repaints which is working fine. However when moving QSlider it will cause repaints and the fps of the application will drop.

The test application will print the timing between frames to the console. When the slider is moved the time increases up to 300ms. When the slider isn't touched the time is ~15ms between frames.