it seems to be mainly related to number of faces in model, although I'm still getting some strange results...
The masks created are totally wrong, not just a little approximate. Entirely different areas are masked out. I shall build a project from scratch and test again.
thanks again for the help
EDIT
hmm maybe not just faces but bounding box size? If I make two models, each decimated to 500 000, but with different bounding box sizes, the processing time for mask from model is radically different.
I think I'll email support...