I have never had any scrolling lag issues on Firefox (in the form of Fennec from F-Droid) on GrapheneOS.

Native Firefox on native Android (or at least the Samsung variant) here and I haven't had issues either.

I've had the issue on multiple phones. Firefox nightly seemed to fix it for a while but it suffers from it too after a while. I need to test if it's UBO or the sister comment that says closing tabs and restarting helps. It's only on certain sites I really notice it, maybe JS or video heavy sites. NYTimes in particular.