Voxel Buffer computes cloud shape at a limited (user definable) resolution and limits cloud shading to that area, so it does sometimes miss smaller bits of cloud. You can increase voxel cache resolution, but this will of course increase render time. It may still be a time savings, so it's worth experimenting with.
I assume that you're using GI caching with all of this, certainly that's flicker stopper #1!
In any case provided the settings are the same, there should be no such unexplained differences between machines for render time. I think it would help troubleshoot if we knew 1: the specifications of the systems involved, 2: the location of your render files (i.e. on a central file store remote to all systems, on a folder on one machine that is also a render and then being shared to others, etc.), and 3: what the configuration of your network is.
- Oshyan