Acquia tested Drupal performance using PHP only, PHP with APC and Zend Server with Page Cache. They find that Zend Server with Page Cache enabled increased Drupal performance by 2605% (in terms of page requests, valid for Acquia's testing environment).
From the whitepaper (Feb 2010):
Effectively, using full page caching based on a GET variable results in performance equivalent to that of static HTML – an astounding 26(!) times faster than plain PHP. While full page caching cannot always be applied, using it where possible can make a remarkable difference. If this approach is implemented, it is important to configure Zend Server so that it will not cache pages that include user-specific content.
- Pimping Drupal
- Zend Server, which some may find 'bloated'
- Requires VPS or Dedicated Server