I always check Opaque in shadow. Ultimately speaking, I wouldn't think the render would be slower in the look run, since it should require less hair density than otherwise, I think. And less hair density = quicker renders.

I wouldn't give the skull a material. That's as much hassle covering up and hiding as the skin, unless you're only going to render distance shots. Otherwise, you'll need to up your density. (Or have thicker root widths, but I'm not a fan of that solution, and I don't think it would work as effectively as higher density.)

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3