In developing the recently released NEVictoria2, SEVictoria and WVictoria sceneries, I have generated a large number of object tiles, each composed of many other objects. These tiles are non-overlapping.
Through experimentation I have found that each object tile needs to have less vertices (in the .c3d file) than around 8000, otherwise there is an impact on frame rate.
Consequently, I've had to implement subdivision of the object tiles which exceed this limit, so that all generated object tiles are within the limit. In some cases, one tile might be 10x10km, in detailed areas the size gets as small as about 1x1 km.
Rendering the objects does not have a significant impact on frame rate. It is the vertex count of the tile that the glider is currently inside the bounding box of, that is significant. Bounding boxes are drawn in the landscape editor in 2 dimensions, so it is fair to assume that Condor 2 uses these bounding boxes to determine whether to perform collision detection against an object or not.
Now, here's the issue: it appears that the glider height doesn't have much effect on the framerate impact of large number of vertices --- the glider can be several 1000 meters above the objects and the framerate is essentially the same as when the glider is close to the ground.
So I suspect that there is either a bug or a shortcoming in Condor 2's bounding box calculations in that altitude is effectively ignored when filtering objects for collision detection. If this were to be fixed, higher framerates could be achieved (for all landscapes employing objects).