My graphic cards maximum untextured triangle performance is 280M/s in case the triangles share vertices, and about 133M/s in case the triangles have independent vertices. Maximal vertex transform rate is about 400M/s.
This means, if the landscape would be visualized using splats, each rendered as single triangle, then at least 8M triangles would be required. Without any culling, this would lead to a performance of about 133/8=16 fps. Here, perhaps the geometry shader might be used to accelerate the rendering. It would be possible to send only one vertex from which the geometry shader generates a quad or triangle.
I case we would visualize each voxel inside the landscape using conventional polygons, we would have to use at least 2 triangles for each to create a quad. This means, taking shared vertices into account, We would have to render at least 16M quads, resulting in a theorethic frame rate of 280/16=17.5.