Pressflow is Drupal’s distributive that provides enhanced efficiency and scaling. It was developed by the guys from Four Kitchens Company.
This distributive was developed to be used on high load websites. Only stable versions of Drupal are known to be used on serious sites as such sites' upgrade seems to be a special process. Pressflow is a product which is developed on the basis of stable versions of Drupal, and quite often new Drupal versions contain the groundwork from Pressflow by default. Due to the developers, they try to dedicate the corresponding Pressflow version to the moment the new Drupal version is being released. The modules compatibility is being considered hereby - what should be in Drupal core, what’s to be left from previous Pressflow release, plus the scope and kind of addons that should appear available.
It's the constant advance of the project what makes us especially happy.
So, what’s useful in Pressflow?
1) MySQL Optimization.
2) Support of MySQL replications.
It appears to be a key feature for big servers.
3) URL Alias cache module.
The module that secures aliases caching seems to be quite handy, too, for it decreases the DB workload. On some projects we observed the 40% decrease in the number of requests.
4) External cache of pages
5) Support of reverse proxy