env fog and motion blur

[super gnu]

so i made some clouds for my sky.

added moblur to scene.

renders ok for the majority, about 40 mins a frame. get to a part with a fast turn, hence more blur (but not super much really)

has done 1 bucket in 10 minutes and estimated frame time is 72 hours.

any tips? for various reasons i have to do all this -in render- as it were. no post or comping.

[super gnu]

ok so i gave up on that. i rendered out a spherical camera of the clouds and sky. mapped back into the environment slot. trouble is, in order to get it to work, i have to render it without exposure correction, so when i map it back, it still looks the same.

unfortunately rendering a total white-out takes AGES. (as in 15 hours estimated, for a 6000x3000 map, with exposure it took 1 hour)

i never understood why rendering overexposed things takes longer? is there a way to speed this up? surely its doing the same calculations but just with higher final pixel values? also, since i didnt finish a render of it yet, any potential problems with file types not handling such an overexposed image? hdr will be fine?

[super gnu]

hmm gonna put this question in a fresh thread cos its not specifically related to env fog or moblur any more.