UPDATE: I've figured something out, though it is not a solution.
Something I've learned about the way that terrain and buildings are handled under the hood: The SDK renders far more than you can see. Specifically, if you are using a local scene with extents, it renders everything in and around the extents you provide, and then it clips every pixel that does not fall within those extents inside of the shader. It does appear to use frustum culling to some extent, so if you look away from the map entirely, it will no longer render.
The performance drop comes from the fact that for every pixel of every triangle that the map is trying to render, all shader code is being executed to set up the color of the pixel, but then extra code is performed to set the alpha of the pixel, which then gets clipped if it's below a certain value. This means that every triangle drawn is doing extra work regardless of whether it is visible or not. By removing the node that connects to the alpha on the tile shader, the performance becomes almost perfect at the expense that nothing is clipped anymore, so the extent you define just tells the map where to focus detail on, but also shows everything around it, too. I also noticed that the shader responsible for buildings is not using backface culling, which is causing some slight performance loss.
I don't have a solution to this, but just wanted this in writing in case any of the developers see this.
