All right, after a bit more testing I think that I am indeed going to chalk this up to the antialiasing.
As I noted above, I’ve seen some dislike for FXAA expressed a few times, so I’m hesitant to include it; the recourse for players who–like me–find performance dropping as a result of antialiasing would presumably be to simply turn it off. (But then see my thread regarding in-game toggling of antialiasing no longer working.)
Rdb, Wezu, thank you both for your responses! 