For an initial, quick performance tuning, see on your site:
Administer > Site configuration > Performance
If better optimization is required, you might install one or two performance modules. For example, if the site is on a shared server, try Boost (static page caching for non-logged in visitors), complemented with Authcache -in CR's file or db mode- or Advanced Cache (for logged in members). If on a VPS or dedicated server, there are several suitable options: Boost, Authcache, Cache Router (which includes APC, Memcache...), Varnish, etc. Memcache and Varnish are also especially suitable when load-balancing multiple servers.
For extensive and detailed information, and more options on server optimization for Drupal, see the following selection of resources.
Drupal performance resources
Information and tutorials on optimizing Drupal's performance, speed, and scalability.
General Drupal performance
- High performance - Drupal group dedicated to solutions and approaches for high traffic, high performing Drupal sites.
- Caching: Modules that make Drupal scale - Wiki page for comparison of performance and scalability modules.
- Server tuning considerations - Detailed collection of HowTo pages.
- Compatible with Memcache module? - Boost module issue including a solution to use several performance modules working together for logged in and non-logged in users.
- System: cron and caching - System, a required core module, provides system-wide defaults for storing (caching) web pages to improve efficiency, and performing other essential tasks.
- Throttle: congestion control - Throttle, an optional core module, provides a congestion control throttling mechanism for automatically detecting a surge in incoming traffic.
- Drupal guide to caching [draft 1]  - Preliminary versions, by Steve Dondley, for an introduction to the various caching mechanisms Drupal 6 uses to speed the site.
- Coding Standards and Performance Optimization - Group for Drupal developers, on optimized PHP coding.
- Performance and scalability - Deprecated forum with old threads; for current discussions see the two performance groups, specially High Performance for Drupal sites.
Videos, screencasts and slideshows
Monitoring and troubleshooting