@jessegrosjean Thank you for looking into this. I may have found the culprit among the various apps that had requested Accessibility privileges (Preferences.app > Security & Privacy > Privacy > Accessibility).
It turns out that disabling Magnet.app, a window manager, practically resolves the issue. There is still a suspicion of lag, but small enough to be attributed to the complexity of the editor and stylesheets. (I think). Anyway, the app is usable again!
I will contact the developers of Magnet.app and report back if necessary. Thanks again!