The Authcache module offers page caching for both anonymous users and logged-in authenticated users. This allows Drupal/PHP to only spend 1-2 milliseconds serving pages, greatly reducing server resources.
Please note that enabling authenticated user caching will require modifying how your user-customized content displays on your pages. You should be an experienced Drupal developer if you choose to implement the full functionality of this module.
Also note that this module will not help improving the performance of administrative forms like node edit. However it is useful for speeding up news or commerce sites with a significant portion of authenticated users.