I have a model with tens of millions of vertices, faces and triangles. How can I optimize it
How to optimize tens of millions of vertices + tens of millions of faces + tens of millions of triangular faces
Do you have repeating models in your scene or can models be made from repeating components? If so consider
Also have a read of
As far as I see you use Blender, so this optimization should be done at Blender first.
There are different ways and tools for doing this, here are some basic ones - https://all3dp.com/2/blender-how-to-reduce-polygons/
I believe that Google search would help you even more to find the right solution.
Get another source unless your model is the universe.
More seriously, try to identify the meshes that have the most (faces/vertices) and start by working these. If these models can be optimized (lowered polys) do that. If these models can be broken down into smaller parts that can be reused elsewhere (as clones, instances or thin instances), do that.
The situation is really much worse than you think. Your screen shot shows you have 56,121 objects. That is going to make you extremely CPU bound. In this state, any scene with this is going to be DOA.
You need to start looking at a way to join those meshes, by material. This can be complex. You will need to consider:
What modifiers are on all the meshes? When joining, the modifiers on the one designated to receive the others, are now going to be applied on the others. Any modifiers on the others, that are not on the receiver are going to be lost. You need to apply them permanently before joining.
Do some of the meshes move / rotate / scale? If so, they must be kept out of the joined mesh.
Is there any parenting of meshes? This may make joining children to parents a better way to go.