Are you sure your guest login is actually getting to the resources as opposed to being blocked at the login screen?

I had some great results with jMeter testing, my server barely breaking a sweat until I realized my users were not actually registered and thus were not logging in and thus weren't actually testing what I thought they were testing.

Look at the result outputs from your guest test run, the page return codes. My hunch is your guest test runs aren't straining the server because it may not be serving them anything other than normal pages.