This is very interesting, thanks for reporting! As far as I’m aware, the only recent commit which affects scrolling would be:
But if anything, I would have expected that to make scrolling smoother, not worse! (I switched this particular feature to use ‘passive’ event listeners, rather than blocking event listeners)
As an experiment, I just made a theme here on Meta which removes those updated event listeners. https://meta.discourse.org/?preview_theme_id=260 (Named “Experiment - remove scroll listener” in the hamburger) Does this theme fix the lagging for you?