For a realistic number of strings, and a size of 60 cm on 140 cm, the .obj file generated takes around 400 MB unzipped and 41 MB zipped, but smaller models of around 100 MB can be generated for tests.
I might misunderstand this, but I find that transparencies work quite well on curtains, and renders fast.
I made some damask curtains a while ago, and I was quite pleased with the result.
I made a quick project file that can be downloaded HERE
and I rendered this in 6 minutes Q4 - 1280 x 670 px.
Lots of light shining thru, and nice patterns on the floor.
These curtains are 200kb each, and the size of my textures are from 400 to 950 kb.
Cec