If you are hosting Drupal sites on your server, the following tips will help you to achieve better performance.
1. Uninstall modules which are not absolutely needed.
2. Remove unnecessary contents.
3. Install a PHP Accelerator (opcode cache)(i.e. eaccelerator, xcache, etc). This will require root access to your server. A PHP accelerator will usually cache frequently accessed files, and will greatly improve server performance. Check for compatibility issues before installing cache.
4. Analyse database queries, and look for slow queries. Create indexes for your Drupal database if required.
5. Install and configure memcached. Details can be found here: http://drupal.org/node/44220 .
6. Install advanced cache module, and apply requested patches.
7. In Drupal admin > settings > performance change the Caching Mode to Normal.
8. In Drupal admin > settings > performance change Aggregate and compress CSS files to Enabled.
9. Install and Configure QueryCache module.
10. Set HTTP headers cache options to optimal values.
11. Use Xdebug to find PHP code bottle necks and then optimize them.