----- Problem -----
When running the display list painting code, we have each item in the display list individually lookup all the live selections and try to determine if its frame is selected. This code (nsRange::IsNodeSelected) is really expensive. Expensive enough that doing this for every display item takes seconds on pages with lots of sibling nodes (such as 1000 comma seperated links). In this case I believe we take ~O(n^2) time computing selections, where
n is the number of nodes in the DOM.
Mostly this isn't a problem for gecko because we have display item invalidation, letting us skip this work on nodes outside the range of the selection. However webrender has no such feature, and as such becomes incredibly unresponsive (checkerboarding due to rendering at ~0.2 fps).
Adding more caching to this code has improved it a lot, but we're still orders of magnitude off of where we need to be.