Unity just released an experimental AutoLOD, a system for automatically creating LOD (level of detail) scenes and optimizing their performance. Once again, the entire thing is 
experimental, so expect warts and dont expect support. The extension requires Unity 2017.3 or newer as well as git-lfs and git-submodule.
Details from the Github page:
Features
- LOD generation on model import with sensible defaults
- Project-wide and per-model LOD import settings
- Asynchronous, pluggable LOD generation framework
- Hierarchical LOD support via SceneLOD
Useful classes (for your own projects, too!)
- MonoBehaviourHelper – a way to run coroutines in the editor + main thread execution from worker threads
- LODGroupExtensions – useful extension methods (e.g. GetCurrentLOD)
- TimedEnumerator – a way to control maximum execution time of coroutines
- TextureAtlasModules – automatically generate texture atlases
- WorkingMesh – a thread-safe mesh class
You can learn a great deal more about AutoLOD in this blog post.