Post process volumes number performance weight

Hello, I have a question.
I have a forest scene, and I would like to define a postprocess volume for it.
The problem is that the forest has a particular shape, is not a quad, si I would need a custom shape.
I noticed that I can edit the BSP of the post process, but I can’t understand how a “stairs” shape, or a “curved stairs” shape could help me.
The question is: if I use multiple post process volumes with cube shapes with same settings to recreate my forest shape, will this decrease my project performance?