Seems like bsp index has a bug in QGraphicsScene.
If i add lot's of items along the y axis on top of each other, then scrolling becomes really slow in upper half of the scene.

In the example program it creates two scenes and views. In one items a on the y axis and in the other on the x axis. And the x axis's view is rotated 90 degrees so it looks exactly like the other one.
In one view the scrolling is smooth everywhere, in the other only on bottom half of scrollbar.