Visualizing Layers in Web Inspector
webkit.org
webkit.org
Some operations are just fundamentally more difficult for any modern engine to handles such as animated properties in a container that has group opacity or transparent layers with subpixel anti-aliases text.
A bit of careful tweaking can really improve the performance of a page. The problem is its a lot of domain specific knowledge.
This is a debugging tool. I'm not sure how you optimize for a visualization of how your layout rendered.