Region::union on a list of does N^2 copies of Shape vectors Regions are mutable, but their Shapes are not. Each Shape has 2 vectors which are both copied into a new Shape object when two Shapes are combined. To union a list of Regions into a single Region amounts to copying the contents of the first region's Shape vectors N times! (The second region's shape vectors N-1, times, etc.) The sum of a list from 1 to N is N(N-1)/2, which is for our purposes is order N^2. :( Prior to my mitigation in bug 98800, we performed this O(N^2) algorithm every time we did RenderLayerCompositor::computeCompositingRequirements (which is currently on every style recalc!) This could be solved by making a mutable union which built up a Shape, instead of copying each pair into a new Shape. I've split this off from bug 98800 so we can land a mitigation for RoboHornet now, and fix the algorithm complexity of Region/Shape combining later.
(In reply to comment #0) > Prior to my mitigation in bug 98800, we performed this O(N^2) algorithm every time we did RenderLayerCompositor::computeCompositingRequirements (which is currently on every style recalc!) This every-style-recalc behavior is tracked by bug 84393.