Investigate Tessellation Performance on Landscape

I have been seeing a few answerhub threads about very low performance on tessellation with landscape.

What is puzzling is that it does not seem to change performance whether low or high tessellation is used, or if distance based masking is used to reduce how much landscape is tessellated. Frame rate seems to plummet.

I think we really need to investigate how much work the tessellation shader does on terrain. It may be doing unnecessary work all the time.

https://answers.unrealengine.com/questions/210923/how-to-control-landscape-tessellation-with-distanc.html

A test level with some landscape tessellation is included in CL 3079376