I'm assuming you're using the Quick Render, given the resolution and detail settings. If that's the case, your GI Cache settings might be rather odd and inappropriate for your camera motion. You have a full 180 degree camera rotation, but are using a single GI cache file that is unlikely to effectively cover both extremes. I'd recommend a sequence of GI cache files, perhaps every 10th frame, and blended "interpolate (for animation)" with perhaps 5 frames blended.
Another option to consider is, since your camera movement is comparatively small and does not appear to be close to any clouds or other elements that would show major parallax, you might be able to get away with just rendering a high resolution spherical camera output and then using that in another 3D app to create a simple camera rotation. This would save you a lot of render time. If there's enough movement on the closer clouds you might need to render those to a separate layer with alpha and do some parallax shifting, but still would save time.
More generally speaking I think your render settings are quite unnecessary in several ways. Detail 1 is going to give you higher quality clouds, true, but it is generally unnecessary (though it would help with water quality); in this case since your scene consists almost entirely of clouds I'd actually recommend using Defer Atmo/Cloud, which will give you even higher quality cloud edges, and then use much lower detail (even as low as 0.25, if there's no terrain visible). AA8 is also likely unnecessary unless you're using built-in motion blur; AA quality doesn't significantly affect the clouds (Detail affects cloud edge quality, but only when Defer Atmosphere is not used). Then, your GI settings are extremely high. A Cache Quality of 10 is almost certainly unnecessary. I'd recommend 4-6. Any higher and you probably won't see the difference, but your GI cache files get a lot bigger, and the calculation times a lot longer. As Fleetwood mentioned, the Receive Shadows from Surfaces options are also unnecessary here because clouds are already self-shadowing, and that option only controls shadows from Terrain or Objects, which are not present here.
Hopefully that will help you resolve your problem *and* reduce render times.
- Oshyan