Sorry for being a bit late here. Generally what Tom says seems to be correct. Some notes:
Imagine turning 360 degrees around a square building. At which frame shall i save uhd cache? In the front of the building, at the back or on the side of the building?
As Tom said - you can use the "load+append" option to add new records to the UHD Cache for multiple frames. BUT usually UHD does not speed up rendering som much in outdoor scenes, unless some heavy light bouncing is visible.
Is it logical to make precision 20 as we will wait only for one uhd cache? What can we do for less flickers?
I would say that setting precision to 20 is never logical (it will not make much difference, and the rendering will be probably a bit slower).
When you are using "animation" UHD preset, or loading a saved precomputation, then there should be no flickering at all.
For instance at which number would we have a better result for gi vs aa balance animation?
Sadly there is no universal answer here. It depends on the scene type. Same as with static images:
- lower GIvsAA = better antialiasing, worse GI
- higher GIvsAA = better GI, worse antialiasing
But in most cases adaptivity should do its magic here, so I would leave everything at defaults. :)
I want to make waves with animated noise on the water. Would it work to pre-save uhd cache or should i use still frame computed?
I would try with pre-saved UHD. Even if there are some differences between what is saved in the UHD and what is currently in the scene, then path tracing will eventually solve it.