Khalid Baheyeldin (aka kbahey) has been involved with Drupal since 2003, and focuses on the performance & scalability in Drupal. He talks about his strategies for diagnosing the performance bottlenecks in a Drupal site, and mentions some of the various tools that he prescribes as a solution.

For more information, be sure to check out Baheyeldin's DrupalCon presentation slides and talk titled 2.8 million page views per day, 60 M per month, one server!