Interesting. I can't immediately think of a reason for this, but I don't know all the intricacies of the render process. I will say, however, that my first question is how this effect "scales". In your test the low micropoly detail render is taking something like 45% longer to render, which is a big deal. If that is maintained even on longer/more complex renders, that would be interesting and important to know. So for example if you have a similar scene that takes 5 or 10 minutes to render with micropoly detail at 0.8, would it then take 7-15 minutes with it at 0.01? I doubt it, but it's important to know. If instead it would render in, say, 5m10 seconds, or 10m10s, then it really comes down to a 10-20 second difference *on all renders*, i.e. it is not proportional but rather a set amount of overhead, then it probably doesn't much matter.