How does mod_pagespeed work?

mod_pagespeed improves web page latency and bandwidth usage by changing the resources on that web page to implement web performance best practices. Each optimization is implemented as a custom filter in mod_pagespeed, which are executed when the Apache HTTP server serves the website assets. Some filters simply alter the HTML content, and other filters change references to CSS, JavaScript, or images to point to more optimized versions.

mod_pagespeed implements custom optimization strategies for each type of asset referenced by the website, to make them smaller, reduce the loading time, and extend the cache lifetime of each asset. These optimizations include combining and minifying JavaScript and CSS files, inlining small resources, and others. mod_pagespeed also dynamically optimizes images by removing unused meta-data from each file, resizing the images to specified dimensions, and re-encoding images to be served in the most efficient format available to the user.

mod_pagespeed ships with a set of core filters designed to safely optimize the content of your site without affecting the look or behavior of your site. In addition, it provides a number of more advanced filters which can be turned on by the site owner to gain higher performance improvements.

mod_pagespeed can be deployed and customized for individual web sites, as well as being used by large hosting providers and CDN’s to help their users improve performance of their sites, lower the latency of their pages, and decrease bandwidth usage.