Seems instantaneous on Firefox for me.

Really?! It isn't instantaneous for me, even on Firefox 62 (Windows). Takes some half a second or so.

I'm on Firefox 68.0.2 on Windows 10.

I can click "Show All" and "Hide All" over and over and there is hardly any delay. Maybe you should upgrade to the latest and see if it makes a difference.

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...

200ms on Firefox clicking show all.