On the TechnoSophos blog there's a recent post looking at how the swapping of a few technologies has made for a huge performance jump for a Drupal-based website.
With a clever hack utilizing Memcache, Nginx, and Drupal, we have been able to speed the delivery time of many of our major pages by 53,900% (from 8,100 msec to 15 msec, according to siege and AB benchmarks). Additional, we went from being able to handle 27 concurrent requests to being able to handle 3,334 concurrent requests (a 12,248% increase). While we performed a long series of performance optimizations, this article is focused primarily on how we managed to serve data directly from Memcached, via Nginx, without invoking PHP at all.
They describe how, by just changing out the web server to mginx and a highly tuned memcached installation, they could get huge jumps in response times. They pushed it even more when they changed the nginx configuration to directly interact with the memacahed server instead of having to rely on PHP's interface. Details on how to get this setup working and an overall view of how it works are also included in the post.