Dear lord, don't use 0 voxels! Hah. New clouds need voxels to function.
A test with voxels set to 0 isn't really a valid test, basically (I'm not sure but there might even be an internal minimum since the shading doesn't work without voxels as far as I know).
Multiple cloud layers is definitely taxing, especially with the new cloud shading model. But I think there is also room for optimization in your settings to improve render times and quality without necessarily losing your 5 layers. Although I will say I can only really see 2-3 obvious layers in your original shot, so it may be better simply to work on getting the same effect but with fewer layers. Enable/disable each layer one at a time and see what is really contributing to the effect, and whether you can get a similar result with fewer layers. For example if you have 2 layers that together create a sort of "overcast" look due to their high overall coverage, instead of using 2 medium-coverage layers for this, you could use 1 more dense/higher coverage layer and get a similar result, probably with lower render time.
Anyway for some general advice on render time, first I would suggest not bothering with the Very High GI setting in clouds, especially for a scene like this that is frankly pretty "muddy" in the lighting already. Generally I would advise *using the defaults* and then optimizing/adjusting from there, so I wonder for example why you decided to use Very High GI for the clouds. Did you feel the detail was not enough with default setting of Still/Medium? It's also a bit odd to use Very High cloud GI with a main GI setting of only 1/1, while the default is already 2/2. I would not suggest going below the default, especially not with higher cloud GI settings.
To resolve the noise, it helps to know what the main sources of noise can be. Primarily they are Atmosphere and Cloud Layer shading, which are controlled by Atmo samples and Cloud Quality respectively, but are also affected by your Antialiasing settings. Note that GI does *not* cause noise, so if noise is your concern, increasing GI settings is not going to help at all. Instead, enable Cropping and start experimenting with your Atmosphere Samples and Cloud Quality. Generally I'd recommend a cloud quality of 1 or 0.75, I would not recommend less than that, especially with highly adaptive AA.
In your specific scene I suspect it may be dark and shadowed enough overall that the *atmosphere* may actually need more samples. I would suggest testing this carefully, find an area of high noise, especially an area in shadow, then increase Atmosphere samples to something like 32 and see if it is less noisy. If it's not, then atmosphere is probably not the source of your noise! So then instead take atmo samples back to 16 and increase Cloud Quality to 1. If that helps, the issue is in your clouds. It's easier to troubleshoot this with fewer cloud layers of course, and that's one more reason to start by testing with atmosphere since it's only 1 setting to adjust in a single node, rather than across 5 nodes.
I hope that helps you figure out the source of the noise and reduce render times.
- Oshyan
