Much lower frame rate after Windows 10 Fall Creators update

I typically run on the highest graphics settings, with 12 chunks render distance, and typically stay around 40-60 fps. After upgrading to the Fall Creators Update, I never get over 45 fps, and frequently bottom out at around 0-5 fps.
I've talked to a few other Java edition users, even some with much more powerful PC's than mine who consistently hit 600+ fps, and they bottom out around 20 fps. They even said they'd been having the problem since the Spring update.
Attached is a screen of the lowest I get, and the highest, which only happens when I completely stop moving.
(I ran this test in a brand new world, in 1.12.2, with the same results as the optifine version)