Complex collision is more expensive regardless of the shape or complexity, because its polygon based and not easily deducible from pure math calculations. You should always use simple collision wherever possible.
I personally wouldn’t have a post process volume. If the effect is always going to be the same, then just add a blendable material to the players’ camera, create a dynamic material instance of it at the start of a game and modify a parameter in the material to create the flashing. This is how it’s usually done, as you can just drive the effect off of the damage your receiving, and can therefore just use a regular damage volume.