Interesting, thanks for mentioning this! It seems to be something with my profile, because a fresh profile is also definitely faster (maybe 200-300ms just eyeballing). The latest unbranded build I got [1] also seems about the same if not faster (maybe around 200ms). Definitely not "instantaneous", but I'll definitely track this down.

UPDATE: At least 3 of the culprits seem to be MutationObservers from my own extensions (as in, from extensions that I have written for myself) that have {childList: true, subtree: true}! I guess I will have to optimize them. :-)

[1] https://queue.taskcluster.net/v1/task/Uht_zkkPThu384OAHNPwdA...