Implement a LOD System similar to the Unity LOD

For my project i need a LOD System that has some more functions then the default Unity LOD System. I thought that i could implement my own system. So i started researching but i cant find any good reference of how to implement a fast and stable LOD System. Of course there is the way with a simple Vector3.Distance calculation. But its not very fast when i do it for every object in my scene. How did Unity that? I looked through the Unity dll's but i looks like Unity does the calculation on the C++ side.