Performance Issue in QGraphicsScene's _q_polishItems()

After adding >30000 Items to a GraphicsScene the first draw takes very long time.
That's because it first calls _q_polishItems(), which removes the Itemes one-by-one from the QSet of unpolished GraphicsItems.
(The actual slow instruction seems to be "it = unpolishedItems.begin();". Tested with callgrind).

A suggested fix is attached...