First thing you have to do after each content patch is to make sure your addons are updated and working.
Also update your weakauras if you are using such.
Some settings are quite broken as in they have a unproportional high cost.
The best example would be “compute effects”, mostly noticeable in the Emerald dream zone at he Ancient Bough Portal or some other spots with fog (volumetrics).
Turning this down 2 notches gives an insane FPS boost in these areas.
Another thing would be raytraced shadows. They can kill your FPS fast if too many players/NPCs are around.
Oh and i almost forgot, the renderscale is bugged, don’t set it to anything other than 100% (native res).
Many Graphical options are not working correctly and the Game demands much more than it should. Even Star Citizen runs better on my PC than WoW. The Few Addons i use are always updated.