It is mostly par with Chrome now pretty much everywhere.
Well browsing most website should not trigger lag in any browsers.
But if we look at benchmarcks:
https://www.phoronix.com/scan.php?page=news_item&px=Firefox-...
SpiderMonkey should get around to jitting generators, but I have a hard time blaming the team for focusing on real-world improvements first.
That is benchmarcking most used ALEXA websites. It's named TP 5 and TP6, mozilla use it internally (e. G to measure progress on stylo) Why is the real world (TP5/6) not public?? Phoronix has never used it, mozilla has never publicly published benchmarcks. I guess they favor chromium but I've never seen all results only the few mozillians posted on bugzilla. Let the world know, publish the results, even if it favor chromium. Anyway the mainstream will never be affected by that public information (most people have difficulties discerning a search engine from the concept of browser, let alone talking about benchmarcks)
Maybe there are people (ab)using the async stuf do do something else, but at least it's not really common.
I'd be curious to see how 68 versus 69 versus 70 compare.