For rich interface websites for first-time visitor to your page may have to make several HTTP requests, but by using the Expires header you make those components cacheable.
PDO (PHP Data Object) is the lightweight and consistent interface for accessing the multiple databases (even of different types) through PHP.
Deploying your content to multiple, geographically dispersed servers make your pages load faster from end users perspectives.
Most of the end user response time is spent on downloading images, scripts, Flash, components, etc. This might be the problem while dealing with the Rich Interface websites in which developer require number of different images, scripts and CSS files.