"Also, the HDRI should not get the blame for the 20 minutes rendertime as rendering without the out of focus blur took only 6 minutes in Final." If this is just for a still image, why not just fake the DOF in post? The results would be just as convincing, and render time wouldn't suffer as much. ;-) By the way, my posts above about HDRI and motion blur/DOF were directed towards use in animation, where low sampling of GI combined with motion blur causes unsightly flickering. For a still image, it really wouldn't matter.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.