There are a few typical difficulties that can cause your site to perform slowly.
The majority of slow-loading websites are database-driven. Unless you have caching enabled for this site server, database content is not kept on the web server. If no caching is enabled, the communication between the web server and the database server creates content dynamically for each visitor.
Concrete CMS software.
Cache & Speed Settings in Concrete CMS software.
Caching Overview of Caching
Article about Drupal Community caching, speed, and performance
“Boost” is a Drupal caching module.
10 ways to boost your performance
10 Ways to Increase Your Speed Joomla
Performance tuning for MediaWiki
File cache for MediaWiki
MODX may be made to fly in seven different ways.
Overview of the phpBB Cache
phpBB cache is being cleared.
WordPress articles about WordPress optimization
Zen Cart performance optimization
Optimizing a private server
Many of the suggestions on this page can be applied to private servers, however for VPS-specific advice, see the PS Optimization and VPS Troubleshooting pages.