Sometimes (La Femme eyebrows, for instance) a mesh need to be hidden using a fully transparent material. Is there one node setup (pbr, vs poser, for instance) which renders faster?

A lot of times in Poser, I'm bitten by the fact that fully transparent does not mean no specula. A lot of times, I have la femme's hidden eyebrows reflecting light. So I turn the roughness to 1, specular color to black. emissive to black (even though it's set to 0 as a value). Fine.

But this left me wondering how the rendering engine treats that mesh. Is it smart enough to look at the values and say "drop these faces; they affect light not at all" or does it leave the faces in, run through the values, produce no affect on the face (but still do the math)?

Is a poser root better for this? Or a cycles root? This is using superfly. How does superfly handle faces with a node setup that produces no affect? And what's the best node arrangement for that?

Maybe some testing? But I was curious if I could get an answer from someone who knows how the rendering engine works.