My five cents:

According to Uwe (and I believe him) clipping code in Qt sucks completely If you can, avoid clipping and rely on graphics view's indexing by dividing your polygon into smaller polygons or do your own vertex based indexing and try to use it for clipping. But it might not be worth the effort - dividing the item should be simpler and you would only do it once. You can also try those brand new caching mechanisms in graphics view, but they might be ineffective when you start zooming.