Hey!
Are there any significant or insignificant performance drops with this mod installed? For example, I want hostile mobs to spawn only in caves.
Quote from the documentation: "true if we are in a cave. This is a more expensive test that tries to test if we are in a cave by checking if the block in all six directions is one that can occur in a cave."

How much more expensive is this test?

How can I answer this in numbers? Every thing that has to be tested obviously adds some performance penalty. However, I can't give you any numbers. This is something you have to measure. In general performance is not an issue with this mod though