The renderer is struggling with so much overlapping and intersecting polygons created by the displacement you have. In the 3D Preview you can see some telltale signs that this is happening. The polygon sizes and shapes are very irregular. While this isn't always a problem, it can be a warning that the renderer is dealing with some extreme displacement.
The main thing that's causing this is the small patch size on the compute normal (1) combined with rocky displacements that have a much higher amplitude (5). Increasing that patch size to 5 gives a much more well-behaved surface. I don't know if you'd be happy with the resulting shape because it's less dramatic (but in a good way for the renderer).
Looking at the overall shader network, I think you could probably put all of these displacements after the Compute Terrain. Compute Terrain also computes normal, so if you have something that requires a computed normal (such as this lateral displacement) then as long as it comes after the Compute Terrain it will work. By only computing the normal in one place you will greatly reduce the render time as well. However, using a shader after the Compute Terrain will mean it works with different texture coordinates. But depending on what you want, this might actually be better at producing a realistic rock face.
Matt
