On MacOS (tested with 10.11 and 10.12) it seems the entire windows/container is repainted despite only a child widget must be repainted.
This behavior decrease the performance of the whole application when the graphic container is maximized or enlarged (some events are even lost)
In the mini project attached, while the white widget has a small fixed size, its refresh rate decreases when the widget container is enlarged.Tested on MacOs 10.12.4 and 10.12.5.
The bug doesn't occur with Qt5.15.2.
My guess is there is an issue with clipping information (which is correctly passed with Qt 5 and not with Qt 6)