If your site is running slowly, there are a few common issues that can cause slowness to occur.
Most sites that run slowly are database-driven sites. Database content is not saved on the web server, unless you have some sort of caching enabled for this site server. If no caching is enabled, content is dynamically created for each visitor by the communication between the web server and the database server.
Optimizing a private server
Many of the suggestions on this page can apply to private servers, but make sure to also check the PS Optimization and VPS Troubleshooting pages for VPS-specific suggestions.