What the guy above me said, plus:
WoW is a HEAVILY CPU bound game.
You can run full Ultra settings at native 1080p (I.E. NOT render scale 200%, which is running at 4K) on an R9 380 or GTX 960. WoW is just not that demanding of a game on a GPU.
Its a CPU issue, and not one that is fixable. WoW is heavily single-thread dependent, because of the nature of how draw calls are handled on any client/server game where the server has to OK what is going on on the screen (which is why mkst other MMOs are also heavily CPU and single thread bound, its not an engine failure, but a reality of how client/server server-dominant code works) - there are people with massively overclocked 5.5ghz+ rigs with custom liquid cooling and even hey cant maintain 60fps at all times in hevaily crowded areas. And like the guy above me stated, Garrison is phased, so it is literally more crowded than capital cities.
No amount of GPU power will "fix" it - you could have quad sli Titan Xs and it wouldnt matter.