VRayBlendMtl question

[duke2]

I have Material A - a complex shader setup, and Material B - a baked version of this. Why is it that, when I put Mat A in the base slot of a BlendMtl, and Mat B in slot 1 with 100% opacity, it renders ALOT slower than if I were to just use Mat B on it's own? Why is VRay doing any material blending/extra calculations at all if it's 100%?

[duke2]

As a follow-up, swapping it does the same thing - ie. Fast Mat in base, Slow Mat in slot 1. Now what's really confusing and annoying, is even if I make it's opacity 0 (black) and the slot 1 mask % 0, it's much much slower than if I clear out that slow slot completely.