Drupal performance and scalability

I've been doing some research and testing with Drupal measuring performance and scalability. Drupal has built in CSS and JavaScript aggregation (now in D6) to minimize the number of http requests, and caching via the database. In some cases with a high amount of traffic caching with the database is not enough. Modules like Cache Router allow the site caching to be handled by an engine other than the database to increase performance for larger amounts of traffic.