Right now, profiles show that the connecting of the tree UA widget causes us to call _ensureColumnOrder in tree.xml, which requires constructing a boxobject for the tree, which trips webrender, which blocks the main process for a significant proportion of time (easily 120-250ms on my machine). This was previously tracked in bug 1508213. It's unclear if this has regressed or if there's just more optimization to be done, but we'll still pay a similar-ish cost for initializing webrender at a different point. Either way, we should be able to avoid this by constructing trees at runtime instead of having them hardcoded in the markup.

I this profile https://perfht.ml/2GTMeO1 this bug represents a third of the time it took to open a new browser window.

(In reply to Florian Quèze [:florian] from comment #1) > I this profile https://perfht.ml/2GTMeO1 this bug represents a third of the > time it took to open a new browser window. Seems that most of that is just that we're forcing the creation of a compositor and it uses sync IPC, which in this instance took a "long" time... but I expect we'd have to pay that cost somewhere else if we fixed this bug.