transform feedback + glDrawElementsInstanced

[aqnuep]

And... this is supposed to be fast? Using a geometry shader and performing per-triangle frustum culling/LOD selection, while using transform feedback? How is this faster than just rendering the models using traditional CPU-based methods of whole-object culling and LOD? You have this whole read/write/read loop going on in the shader. That requires an additional buffer just to write this intermediate data that you then render.

No, nobody said that. You perform per-instance or per-object frustum culling/LOD selection using a geometry shader. That's orders of magnitude less work than the actual rendering.

Also in general, when I think performance, I don't think geometry shaders.

While using a geometry shader does has its cost, it's not the evil itself