Do you have an example of your "pet naive unoptimized UI engine" ? Or some code to show ? :)
My point wasn't about how good my pet engine was (because it wasn't), but about the fact how the overcomplicated model of the HTML-CSS-DOM-JS makes it so hard to optimize web page rendering that even an unoptimized prototype UI engine outperforms the browser.
I am amazed by the people who develop browser engines because they do something almost magic of fitting that monstrous dynamically modified DOM into so little memory and making it so fast.
I conjecture that in a naive unoptimized implementation a web browser would consume gigabytes and took a couple of minutes to load an average page.