It's your atm/cloud/Render settings. They are way too high in general. For the 2D Cirrus lower the samples to at least .6, maybe even lower. Also consider going with Conservative Acceleration for your Cirrus layers. None is overkill. I would also lower the main atmosphere node samples to say 32. Deeper tones in the atmo I find do increase the render times a bit, but certainly not this much, you need to lower your settings. Another observation is rendering a Detail of 1 is overkill, that ain't helping your render times (.7 -.8 @ max). I cropped the sky rendered in 6 min with below setting adjustments
Atmo 32
Cirrus - .5 Samples w/ Conservative Acceleration. You can even drop that a bit (Both layers)
Render .75
AA 6
Defer - Off