Testing locally, small-group-opacity-2500.svg seems to take 5x or so longer to render when SVG display lists are enabled. I thought at first this must be because the group opacity optimization wasn't working correctly with display lists, but in fact we can't even use that optimization for this test, so that's not it. Needs some investigation.
Created attachment 647771 [details] [diff] [review]
For the record, the problem is that when SDL is enabled, we create an nsDisplayOpacity item to handle the group opacity on all the rects. When painting through that we end up under BasicLayerThebes::PaintThebes where we call BasicLayerManager::PushGroupForLayer, which calls gfxContext::PushGroupAndCopyBackground. The PushGroupAndCopyBackground call spends virtually all its time in _moz_cairo_paint is much more expensive than the PushGroup call (which doesn't call _moz_cairo_paint) we make in the SDL-disabled path. In fact it takes up 80% of the paint time. A PushGroup call in comparison is virtually free.