If it's all that transparency in those layered clouds then let me suggest the
following.

Intersect two planes that are seperated about the distance you have all those
planes at now.  So say you want it in the region between y500 and y600:

intersection {
plane {y, 500 inverse}
plane {y, 600}

pigment {color rgbf <1, 1, 1, 1>}

interior {
media {
scattering {1, 1 extinction .1}
samples 5, 10
density {
your pattern stuff here
}
}
}
hollow
}

Having your media defined to such a small range will generally render pretty
fast.

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