Level of InterestThe client sends camera location updates back to the server. The server then optimizes the client stream, first only stream dynamic mesh for objects very near to the camera, and reduce object transform update as an object becomes more distant to the camera.
Level of DetailTo have the scene load as fast as possible for the client, the server streams the lowest level of detail LOD's to the client first, and then slowly sends the higher resolution mesh and textures. LOD's are generated and cached on the fly by the server and could be adaptive to target the clients profile (mobile or desktop).
For each generated LOD the reduction in triangles can be set to any amount, because we do not attempt to preserve UV mapping or sharing materials and textures among LODs - instead we use Blender's decimate modifier to reduce the mesh, uv smart project to generate new UV's, and then generate a new texture using Blender's bake with "selected to active object" using the high resolution mesh as the source of the bake. (baking down to a single material and single texture provides client side performance gains as well, by reducing GPU material/context switching)
Three.js has another feature we can use to improve LOD, subdivision surfaces! This puts no extra bandwidth strain on the server because the subdivision is all done on the client side. Note that in this video the frame rate drops on subdivision because the subdivision is recalculated every frame, this could easily be optimized to only recalculate when crossing the LOD distance threshold.