I ran some tests, based on Circular control used for walkable areas control . For this I added a lot of meshes in an example. Much more than my room has. For this purpose I added .toNonIndexed() to the BufferGeometries as a test, because my room elements are mostly of this kind.
This test arrangement runs quite smoothly. Only when the mirror is in the field of view of the camera it gets a bit jerky. Try it there.
AreaControl
In my room I took out the mirror - no improvement.
The strange thing is that the jerking in different places is different. But sometimes strongly, where only two or three walls and floor, ceiling are close to the camera.
One guess is that the many commands .addGroup( …, 3, … ) “fragment” the geometry. However, this does not fit the behavior when only two or three walls of two triangles and one material each are in the camera’s field of view.
Combining the material groups for my room is very complex and only worthwhile if it is really a solution.
What is the background to the fact that the material groups are created using elaborate script objects { start: integer, count: Integer, materialIndex: Integer } and not via BufferAttributes?
Still baffled. 