When we detect a preserve3D stacking context, we always render the 3D pictures into separate surfaces and then draw them in proper order in the alpha bucket. There could be an opportunity in some cases to recognize some of these pieces to be opaque (and axis-aligned), so that we could draw them earlier and without blending.

I would expect this to be a minor optimization with medium effort. We discussed this previously, and bug 1584794 is what triggered me to file this one.