If you have looked at QImage in Qt4 you'll have noticed that there is the typical QImage::Format_RGB32 and QImage::Format_ARGB32 plus QImage::Format_ARGB32_Premultiplied.
Maybe you have wondered why is there, well, i'll tell you, it's FAST.
I have done a simple test rendering 100 times gear-flowers.svg wallpaper in a qimage of 1024x768. The results follow
RGB32: 20351 msec
ARGB32: 39273 msec
ARGB32_Premultiplied: 20248 msec
So if you look at the numbers, you'll see ARGB32_Premultiplied is actually faster than RGB32 that is not even producing correct results.
So QImage::Format_ARGB32_Premultiplied should be probably renamed to QImage::Format_ARGB32_UseMeBecauseImFast :D