2015 MBP, running Firefox. I've also experienced the lag on Chrome.
Right now I'm transcribing old Irish-language stories and giving a commentary about various features. I notice if the story gets too long, or if I get too many bullets in the list block, it starts to get super laggy. I've noticed it across both browsers. And the sad part is the "long" really isn't that long, about a page of stuff in Pages/Word.