On the PHP-HTML.net site today there's a new tutorial about creating something that could give your site a pretty big performance boost - a simple method for caching your data.
Cache is a programming concept that can be used in a various range of applications and for various purposes. A cache library can be used for storing database queries for later use, to store rendered pages to be served again without generating them again, or to save indexed pages in a crawler application to be processed by multiple modules.
They include the code to create a very basic filesystem-based caching system for any kind of data you need. It sets expiration times and stores each bit of information in its own cache file to avoid overlap on read/write. This is a pretty simple example of data caching, but it gives you an idea of something that can help speed up your application. If you need something with a little more power, you might check out memcached.