It's faster because of the detail of the mesh of the planet at the heightfield area.
As I've stated several times, you could have hundreds of shaders slowing TG down, or just a few, but as soon as it's a heightmap and simplified, it's far faster. At the cost of geomerty detail. Hence adding displacement, or lateral dispalcement, etc, is just a mess of folds and tears. Applying displacement to Classic Erosion of Heightmaps is a pain.
PS loading a ter seems to work fine but not sure of impact. Rendering now.