I recently noticed that repeatedly changing the texture clamping setting of a texture can bring down performance quite drastically.
In one scenario I had a 20% performance difference between using duplicates of each texture with different clamping settings or reusing the same texture and changing the clamping as needed.
Can anyone explain why this is such an expensive operation? I would have thought that this might have the same impact as changing one or two uniforms per primitive but it is considerably more than that. Is it really necessary that such textures have to be doubled to get optimal performance?