We’ve migrated from core Drupal-6 to Pressflow, a back-port of Drupal-7 performance features. Using Pressflow allows us to cache anonymous web-requests (about 77% of our traffic) for 5-minutes and return them right from memory. While this vastly improves the amount of traffic we can handle as well as the speed of anonymous page-loads it does mean that anonymous users may not see new versions of content for at most 5 minutes. Traffic for logged-in users will always continue to flow directly through to Drupal/Pressflow and will always be up-to-the-instant-fresh.
Read on for more details about what has change and where we are at with regard to website performance.