To put it simply
Wow's graphic's engine is old, they may overhaul some of the graphics to make it look better with each expansion but the backend driving it all hasn't really changed since the original came out. its very old and it's showing its age with crappy fps for effects that newer engines can run without effort
At what settings are you running it? Remember, if you ran it on high settings in Cataclysm, you would have to run it at medium now to achieve the same frames per second: they improved graphics on ultra setting, and old ultra became high, and old high became medium.