I was literally in the middle of typing out this long-winded post about all the things I have tried and how frustrated I was - when I solved the problem. For me, anyway, and hopefully for some of you.
Frame rates sucked, yes, and I had the latest drivers (417.22 for nVidia). I tried all kinds of stuff, toggling DX11 and DX12, toggling the video card to use - and still, even on graphics setting “1”, I could only hit about 50fps.
And then the tech in me said, “Don’t start in the middle, start at the beginning. This wouldn’t be the first time an addon caused frame rates to take a nosedive.”
So, I did a UI reset. Bam. 100fps (60 with VSync enabled) on graphics setting “7”, which is what I was at before the patch. It’s worth noting that the game stopped crashing on exit (happening since the patch) when I did the UI reset as well.
https://us.battle.net/support/en/article/7549 if you need it.
I will edit this when I find the offending addon. Good luck all.
Edit: WeakAuras 2 (2.10.5) was causing my frame rate to absolutely tank. I have four options under addons for WA2, and even enabling just the main WA2 while leaving the Model Paths, Options and Templates unchecked caused my issue.
Good news is that WA2 is updated regularly, and the issue has already been reported on their Curseforge. You can also revert to 2.10.4 to solve the issue.
https://www.curseforge.com/wow/addons/weakauras-2/files