WoW: Cataclysm - DX9 versus DX11 - huge performance increases!
WoW: Cataclysm DX11 performance increases - up to 30% faster!
If you're an owner of DX11 hardware (AMD Radeon HD5000/6000 series or NVIDIA's GeForce 400/500 series) then you're in luck. To enable the DX11 performance increases, you'll need to do a bit of shortcut editing.
You'll need to add '-d3d11' to your World of Warcraft shortcut. Or, open your config.wtf file (located in ...World of WarcraftWTF) and add the line SET gxApi "d3d11".
The increases are anything from 10% to 30%. At CPU-bound resolutions (1680x1050 and 1920x1080) there are some large benefits (roughly 30%!!) by enabling this feature. At 2560x1600 the benefits are not as large, but still 22% extra performance is nothing to sneeze at.