WordPress is a very powerful program and sometimes WordPress sites will not load as fast as desired. This could be simply because your website is getting lots of traffic, however there are still steps that you can take to help speed up your website.
1. Keep WordPress Updated
The Wordpress developers are always making changes to the software to make it run better and more efficiently. Most of the time, WordPress can be updated automatically when you log into your admin panel.
2. Keep track of your plugins
It is important to keep your plugins updated, but it is also important to not have a lot of plugins installed that you do not use. For each plugin that your site has to load, it can slow down the overall speed of the website. Also, there are some plugins that are known to cause potential slowdown on a website that we recommend to avoid unless they are absolutely needed for your website. The following plugins specifically are known to cause issue with load time: wp-forum, wp digg, geo-mashup, and wassup.
3. Use Caching
We recommend the use of WP-Super-Cache as a caching plugin for Wordpress, which stores your Wordpress pages as static HTML files to increase the load time. From our own studies, we've seen that WP-Super-Cache can reduce CPU and Memory usage by up to 70% and increase site loading time by up to 50%. Here is more information for installing Super Cache: http://wordpress.org/extend/plugins/wp-super-cache/
4. Do not have too many posts on one page
The homepage of your site will have more hits then the rest of your pages. To help your homepage load quickly, it is best not to have too many posts showing on one page as the more posts there are to load the longer it will take. The content of your posts is also a factor in your site's loading time as some types of content such as pictures or movies will take longer to load.
Your theme is just as important as the content. If your theme is heavily based on images and large flash headers, that may be causing your WordPress site not to load quickly. Keep that in mind when choosing a theme, as while it may not impact a site with low to medium visitors, one with a high volume is likely to be affected.