Ajax calls are just like any other HTTP request that might be used to build a web page. However, due to their dynamic nature people often overlook the benefit of caching them.
Rule 14 of High Performance Web Sites states:
Make Ajax Cacheable
Make sure your Ajax requests follow the performance guidelines, especially having a far future [...]
Google has launched a new site that promotes the use of techniques to make web pages load faster. The site shares some of the tools and ideas that are used within Google to optimize its own web sites.
The article on caching uses HttpWatch to show the difference in performance when the Expires header has been correctly set:
When you setup a web server there are generally two types of caching that you need to configure:
HTML resources are expired immediately so that any changes made to a site are quickly picked up by existing users.
You set everything else (e.g. images, CSS, Javascript) to expire at some distance time in the future.
This caching scheme is covered [...]