Render element-trees lazily

[sachinhosmani]

Right now, on big pages (e.g. Topeka) it takes about 3-4 seconds for the element-trees to get rendered.
It is not the DOM traversal but the tree rendering that takes most of this time. I feel there is no real need to expand the entire composed DOM tree. We could lazily render element-trees as and when they are expanded IMO.

[addyosmani]

👍 This is painful on any app with a significantly complex element tree at the moment.

An alternative option to lazily loading (which I think we should do) is rendering the initial element-tree and on paint completion begin loading up the rest of the composed DOM trees in the background. This may improved perceived performance on expansion.

That said, I'm unsure how much blocking behavior that operation may introduce into the UI. Just something to consider.

[sachinhosmani]

Can you explain how we can run rendering code in the background?
I thought it is always blocking.

If we did decide to load element trees lazily the search button mentioned in #41 is a must because it will be very difficult to find a deep element without it if the tree isn't even fully expanded.