Display Normals colours very slow compared to display a Heightmap. Why?

Can someone explain why display Normal colours is 4x more computational heavy that calculating a heightmap, given the same tree length? One would presume the same steps are involved.

Any suggestions on an alternative method to display the slope angles?


Screenshot 2022-08-12 at 22.54.05
download to .gh file (external download due to exceeding max upload size: download (25mb)