Yes, you are right. I planed it to be used only as a preview technique. The calculation of the normals in my shader is very expensive and eats the framerate for breakfast.

The problem with cpu deformation is, that I need to use cpu bmap lookup. There I loose the speed I gained by generating the heightmaps with a shader.

Perhaps the terrain preview will only be semi realtime, when you release the left mousebutton to set the value, not during every frame during value changes.