Originally Posted by
Evangelion
Dike, if you're talking about FPS, it's because of WotLK engine which is quite old, and it's using only one thread/core if I remember correctly, while MoP is using multiple cores, so no matter how good you Graphic Card is, single core usage will bottleneck your card thus lowering your FPS in AV, and multi-core usage doesn't allow that to happen on Cata+.
PlateBuffs and TidyPlates both eat VRAM/FPS as well, I suggest turning that off.
Now, if that's not what you were talking about, my bad!