I started experiencing the same issues at the same time so many of us keep describing.
Technically-speaking it’s related to lower than normal FPS and higher than normal ping or packet-loss (widespread server-related issues), but visually-speaking it’s a sporadic microstutter/hitch that is most blatantly noticed when trying to aim and shoot consecutively while strafing in high-intensity battles and makes matches unplayable. One time it even caused my “rendering device [to be] lost” and crashed the whole game.
I’ve never had to deal with this before and haven’t changed any settings in-game or on my high-end PC. Only thing that could have changed is something on Blizzard’s side, approximately a week ago transitioning from the Anniversary event.