Hi,
You may use a Level of Detail actor, and also use some Geometry filter to extract only the outside Layer of a Bulk Mesh.
Using Mesa+LlvmPipe gives quite good results. Please try the gallium driver for that.
Also notice that if you set your actors to double side rendering is slower than only set them with one orientation.
Other thing it might be useful is doing culling, if you haven’t already. That quickly discard polygons in the same orientation than the camera, unless you use double side rendering.