What device are you testing this with? Maybe it's just not powerful enough for this. The atmosphere shaders in SGT are highly optimized, and the planet surface shader is very simple if you disable everything. However, there is only so much that can be done since they take up so much screen space. If your display is 1024x768 then that's almost 1 million pixels that must be rendered for the ground, and if you have an atmosphere that's 2 more million pixels because the outside/sky and ground are rendered separately. If your background clouds are made using SGT's backdrop feature then you may want to disable it, as that's probably another full screen rendering. You can enable the Overdraw rendering mode in the editor's Scene view to see how many times each pixel is being rendered.