I am rendering a scene with a static shadow map (i.e. the shadow texture is rendered just once at the start of the level). I use the same technique as the shadow mapping example from the SDK. When I render my scene the render time increases from 30ms (with no shadows) to 50ms. It seems that the texture2DProj function in the shader is responsible for the extra 20ms render time. Is that to be expected?
Actually, nevermind that post. I was stupidly multiplying matrices in the shader. When I premultplied matices on CPU the render time drastically decreased.
Glad to hear you found the cause of the performance difference