Yeah, with graphics on high, it will be rendering:
7 layers per voxel slice, each with a texture of 1024x1024 ARGB.
There are 20 voxel slices.
So this is 140 1024x1024 ARGB textures, 20 of which, have a 2nd texture blended with them.
That's essentially all it's rendering, and I can get about 500FPS on a relatively good computer. (My laptop gets like 100FPS.)
The map generation just takes time because it's manipulating each of the 1024x1024x20 voxels, and assigning them a colour. (It also saves each voxel slice as a .png for map loading/saving as well.)