Shader Performance Precise Question

Helo :3

I'm trying to optimize POE as best as I can. One thing that keeps me from getting clean/smooth performance are shaders.

I know, that shader bar in graph means "shader caching" BUT is it:
-compiling shaders?
-saving shaders to drive?
-reading shaders from drive?
-all of the above?

I tried 3 different engines: Dx12, Dx11 and Vulkan. Best overall performance is on Dx12, but it seems like it caches shaders non-stop for example: when I sit in hideout for about 10 minutes using skills, walking around and making sure that shader bar won't move a milimeter -> tp to other area -> tp back to HO it caches shaders again, but it doesn't say what it does exactly. My hope is that it doesn't compile them again cause that would be faking stupid.
I just want to know what can I do to make my game work better and solve issue with shaders.

Before anyone will try to say "change your gpu set.." that's what i tried:

cache size to 100GB -> delete cache -> compile shaders -> delete shaders -> set size to unlimited -> compile -> delete -> set cache folder to be on the same drive as game -> compile -> delete -> set size to 100GB -> compile.
Did that for Dx12 as well as Dx11. Also different performance modes etc.

I don't want to know the answer for my computer specifically. I just want to know what that funky bar means, and how shaders are behaving in this game.