Hihi, fast volumetric...
Sure, volumetric atmosphere in itself doesn't take much more rendering time.
Things change, when you want to take full advantage of that volumetric environnement,
using global lightings (ambience, illumination or radiance) with fogs and other volumetric materials...
...dreaming of a lil' renderfarm with some G5 double bipros ;-)

I told an error, above : in volumetric atmosphere, "lens flare" seems to be necessary,
but not the other options of that window.

I have not yet found "my" way to do atmospheres : I load one of the presets, and play with those.

In standard atmosphere, there are those colour map settings.
In volumetric, it depends on which type of illumination one choses :
"Standard" doesn't seem to give much possibilities - or, I didn't yet find'em.
There is the "clouds" tab, where one can group clouds overhead, so clear the horizon, and lower general exposure of the clouds. An the "fog&haze", to vary near horizon.
"Ambient" gives much more playground, to define sky behavior.

Something I don't yet understand, is how and why the "fog and haze" in ambience lighting imports so much on low sun,
even when "fog attenuation" is set to zero :
pull the "fog density" slider to zero, - and the sun's gone, too.
100 to 200 seems to be a "transparent" setting.
Also, the "luminosity" of the fog regulates directly the intensity of a sun near horizon.
Beautiful effects. Don't "repair" that, please, e-on !
Just funny, to find that in "fog" settings...