When using standard settings for my system (no basic shaders, no atmospheric shaders), I can get something like 20 fps on an empty sim with not much to render. If I go and change environment settings, such as switching to default sunrise/noon/sunset/night, that's fine. I can open the Environmental Settings as well without a problem, and drag the time-of-day slider around without a problem. However, as soon as I open Advanced Sky (I just realized I should test with Advanced Water, too), my frame rate will plummet from 20 fps to 2 fps or thereabouts. This persists even after I close the sky editor. It seems almost as if the Windlight atmospheric shaders are somehow kicking in (using them does indeed bring framerate to a crawl), except I see no such effects (no clouds, for one). The sky will change color according to what setting I use, but otherwise nothing happens that seems to call on the shaders.