Use the global content distribution network caching feature of Force.com Sites to dramatically increase the performance of your Sites-based websites. It will also help you avoid Force.com Sites limits.
A public Visualforce page on a Force.com Site may make a number of queries, or even call outs to external web sites. This will slow down the rendering of the page. You can ensure that page is rendered much snappier by configuring the page to be cached.
To cache a page, set the cache attribute on the page to true, and set a value (in seconds) in the expires attribute. For example:
<apex:page cache="true" expires="600">
rest of page goes here
This will also decrease the service request time - the time Force.com spends rendering the page. As the service request time is a limit on which a deployed Site is measured, caching will help reduce this limit.
Force.com uses a global content distribution network. For example, the first user to access the page in the UK will probably not hit the cache. Subsequent calls by that user, or any others in the UK, will hit the cache (provided that the page cache hasn’t expired). Use static resources to ensure other items your page references are also cached - like CSS and image files