Re: [whatwg/dom] Declarative Shadow DOM (#831)

I was testing Chrome's behavior again, and looks like the overhead is now lower vs light DOM. It used to be ~3x but now I'm seeing more like 1.5-2x. (the test ends up with the same number of DOM nodes after parsing the whole page).  Perhaps https://bugs.chromium.org/p/chromium/issues/detail?id=1315283 helped here.
It would be still great if someone could profile the internals of Blink to see where the overhead is coming from? Is it about adopting nodes from the template document to the real document or about creating ShadowRoot or what?
And once we know what the issues are, we might be able to find ways to optimize more of the overhead out - in the spec level too.

The reason why I'm so worried about performance is that since declarative Shadow DOM hides operations like shadow root and shadow DOM creation, it becomes less obvious where the page load performance issues might be. So if pages end up using declarative shadow DOM very heavily, page load times may regress without the developers easily realizing why.

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