Thanks for your reply.
Buit-in QT apps also have this problem, but because the window and/or the actual UI elements are small in size it is less noticable. If you look closely on the Calculator app, you can see how cursor is slowing down too. Perhaps you will start noticing this after watching the video below.
I have made a little vedio to demonstrate the effect:https://www.youtube.com/watch?v=5dRDGzhcnn0
Note how mouse pointer is moving smoothly on the blank area of the window and lags over at controls. It's as if it is going through jelly. If you have more controls on the window, mouse becomes so laggy, it's unusable. CPU load is minimal though.
There could be no error in the example app in the vedio - we created a blank project, put the controls on the form and that's it, it is not doing anything else at all.