As many people have noticed, revision 4.0-155 removed D3D9 as a video backend, leaving D3D11 and OpenGL as the sole hardware backends in Dolphin. For the longest time, D3D9 was considered Dolphin’s fastest backend and was a favorite of Windows users. But then, why would it be removed?
While it was enjoyed by users, it was a source of endless frustration for the developers. D3D9 is inherently flawed, and working around its problems wasted time and slowed development. With D3D9 removed, the developers can focus their effort on making the emulator better instead of pandering to the ever growing demands of a flawed backend. This is why the D3D9 backend was removed.
D3D9: Inherently Flawed¶
Dolphin's D3D9 backend was mostly known for its speed. On AMD and Integrated Graphics cards, it is by far the fastest backend. But Direct3D9 is very old; it was released in 2002! Its age means that many modern features are simply not available for it, features that Dolphin needs for GameCube and Wii emulation. And that's where its speed came from. The D3D9 backend was as fast as it was because it simply didn't emulate certain effects. All kinds of modern functions are simply not possible in D3D9.