Well, we try mask, and Constant Scalar inputs to make Zero density, Altitude offset, Zero Depth.
This help to improve render time but not as much as complete disabling unused branches.
Finally we make animation to move Center of clouds position far-far away from camera.
This helped! Now render time is comparable to render time when unused branches disabled.
Still would be usefull to find more easy way to disable nodes in selected frames.
Multiplexor needed!
Any sugesstion from Matt or Oshyan?