How do I find out which widgets/canvas items are visible?

[jacek]

I call QCanvasItem.update(). Well, it is a subclass where I implement the drawing of the item. Whenever a data even occurs in my system I have to go through a lengthy formatting procedure and prepare the output, then I call update() to get the data rendered.

QCanvasItem::update() only marks canvas chunks as modified. IMO it shouldn't trigger any repainting (the docs say something different, so I might have missed something when I was reading the sources).

BTW. You can try tuning the QCanvas chunk size. Also make sure that your item reports proper bounding rectangle.