The universal settings say to reduce the light cache interp. samples to 5 from 20. Why is this? I have read a lot of tutorials and they all seem to say the same thing. lower your interp. samples to 5 and you will get faster render times. They never actually explain at what cost.
I am guessing that the time increases with more sample interpolation is because they are obviously adding extra calculation time per pixel. that is -Every pixel has to have 20 calculations instead of 5.
Hope this helps