Drupal performace debug

I have a simple drupal 7 installation width 50 modules but something inside it is not working properly. The installation is temporary hosted in a dedicated server with a 4 core Xeon 2.8 GHz, the big problem is that a simple page load burn 20% or 10% of CPU. The problem is obviously for the authenticated users, because the NON authenticated have the page cached. I've tryed removing every modules and i got finally 1/2 % of the CPU on a page load, so the problem is basically stored in one module or in a module setting. I can provide an ssh user and the admin for drupal 7, i've already installed xhprof, devel etc and here we i need someone good in debugging and find where is the problem.