The performance is terrible for me too and I'm using last year's Snapdragon flagship, it feels like it's dropping to something like 15-20fps when scrolling. God help anyone trying to read this on a lower end device.
It is readable but painful on my 7 year old mid-range Android phone.
Silky smooth on both a two year old Snapdragon 8 Gen 1 (Xiaomi 12) using both Chrome and Firefox and on a iPhone Pro Max 15 using Safari. Might be some other problem with your phone?
It seems the issue is specific to Firefox for me, if I switch over to Chrome it's perfectly smooth.
Strange that Firefox works fine for you on a slightly older version of the same platform.
Actually realized I was using Firefox Focus on that Snapdragon 8 Gen 1. Just downloaded Firefox to try it and it's also the same as Firefox Focus...no issue. Could it be an extension? Fwiw, I don't have any extensions installed on any of the browsers.