For large georeferences textured meshes, generally 3D Tiles is used which uses a hierarchy of oriented bounding boxes to support various LODs. If you split up your model into chunks as well as diferent LOD levels, the viewer can then request chunks based on what is in the view port as well as the zoom level. The Cesium implementation leaves a lot to be desired; it't pretty tricky to get right. This will become commonplace since 3D scans are getting more common.