I think that I got my information from this thread; I suggest taking a look at it, starting at the post that the link should take you to, and try the suggestions given there (I gather that the person who posted that linked-to post is speaking from some degree of experience with GeoMipTerrain).
Based on what I see there, it might be worth trying the distance calculation despite the lower execution time on returning “False” – it may be that it has a greater effect than expected.