Vray rendering speed with partially transparent textures

I am observing significant slow-down in rendering with v-ray if the texture has partially transparent areas. This can be seen from the following case. (Both cases have "Use color texture as transparency" checked)

This renders fine (Fencing Diamond Mesh, from default SU materials).

This render has trouble even starting (Fencing Mesh Blue, from default SU materials)

Upon close inspection I find a few half transparent pixels in the alpha channel of the second case, when I replace them with fully transparent or fully opaque pixels I can take renders fine.

I would expect a slowdown for partially transparent textures but not this bad. Am I missing something here?

Ogan

paperball.skp