This tip saved my sanity for DS4 and hair with raytracing: apply the ubershader2 over it, do the 'alt click to preserve current settings' trick if you can. (I forget the exact key sequence, but someone more conversant in DS can likely chime in.)

You can actually leave raytracing on for the hair -- but turn off AO in the surface settings. It's way down toward the bottom of the list with US2, I believe. It's often the AO on hair that makes it utterly murderous on render times in DS4 in my experience. Raytracing stings a bit, but not like the AO does! :)

-D
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It's all fun and games until someone loses an eye texture.