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Blizzard's World of Warcraft 3D engine is said to center on a modified Warcraft III engine, which required DirectX 8.1. With that said, World of Warcraft now employs a DirectX 9 code path, extending compatibility to a broad range of modern graphics cards dating back to 2003.
Of course, gamers get excited about subsequent versions of DirectX (or OpenGL, for that matter) because they often herald a new generation of effects and technologies developers can use to augment realism. But a new API doesn't necessarily have to center on beautiful new features; it can also introduce more efficient ways of doing things already seen today. We saw that recently with Firaxis' Civilization V, which uses DirectX 11 to help improve performance (rather than introduce new eye candy).
As it turns out, Cataclysm incorporates experimental support for DirectX 11, and rather than adding anything visually, it's designed to help speed up performance, too.
Of course, if you want to take advantage of DirectX 11, you'll need a DX11-capable card from AMD's Radeon HD 5000/6000 lineup or Nvidia's GeForce GTx 400 series. You can't enable DX11 through a graphical option. Rather, you'll need to add '-d3d11' to your World of Warcraft shortcut. Or, open your config.wtf file (located in ...\World of Warcraft\WTF\) and add the line SET gxApi "d3d11".
What sort of gains can you expect to see? Much to our surprise, they're significant.
Using a GeForce GTX 580, frame rates jump more than 30% at 1680x1050 and 1920x1080--two resolutions that are processor-bound for this fast card, not graphics-bound. Moving up to 2560x1600 cuts the gain to 22%. Though still not bad, this shows that DirectX 11's mechanisms for improving threading and memory handling can greatly speed up rendering, so long as there is not some other bottleneck limiting performance.
AMD's Radeon HD 5870 sees similar benefits as the GeForce card at 1680x1050. Because it's not as powerful, those gains erode a bit at 1920x1080 and evaporate almost entirely at 2560x1600 once the card can no longer outpace our powerful processor.
In what almost seems like a stroke of irony, it looks like the folks most likely to see big speed boosts at the hands of Blizzard's DX11 code path are the ones with fast processors and near-overkill GPUs. If you built your PC with balance in mind, there shouldn't be as much room for API optimizations to augment performance beyond what you're already seeing. With that said, I've been passing this DirectX 11 flag around to a number of friends with lower-end Core i5 and Core 2 Quad configurations, and they're all seeing roughly 20 frame per second performance boosts. So long as your GPU is DirectX 11-capable, there's a fair chance that enabling Blizzard's code path will at least do something for your gaming experience.