I think I understand why you need to play tighter to be able to beat the rake.
I also understand that some people say you don't need to play that tight, because you also have a skill edge.

Now, when do you start seeing the results of being too loose and rake?

For example, I have had some winning sessions where I played 24/21 and also losing sessions where I played 20/18. So short term, obviously not, like everything in poker.

But how do we actually know what too loose is considering the rake?

disclaimer: This is not to search for an argument to play loose. I am actually making a habit not checking my poker tracker after every session. I just noticed that I always seem to play around 22-24 vpip and 20/22 pfr. So.. that might be too loose, or not.