light cache and 2 sided mtl question

[Evox_Productions]

I am experimenting with the 2 sided material. I am trying to make a slightly transparent curtain. I have a 2 sided material with two identical white base materials. My main issue is when i add a little opacity to the base materials my light cache takes forever to render. Is this normal? Ive tried refraction instead which renders quick but looks crappy and gray. Also the curtain geo has a little thickness to it .

Thanks for any info

[Zdravko]

Make sure the curtain geometry have no thickness