Two years and 18 releases later, Google has removed the beta designation from mod_pagespeed, the open source Apache HTTP module that speeds up web pages using filters to enhance pages by re-writing the page resources served. It is already in use at more than 120,000 sites including EdgeHost, GoDaddy and DreamHost hosting providers. The technology speeds up sites, reduces page load time, and improves Web page latency and bandwidth usage. The Apache HTTP server module automatically applies Web performance best practices to pages and other assets (e.g. CSS, JavaScript, JPG and PNG images) that is serves without requiring any page modifications by the user or impacting the look and feel of the page that is served (although it is said to not play well with WordPress sites). Best practices incorporated included optimizing caching (keep data and logic off the network), minimizing round-trip times, minimizing request overhead, minimizing payload size (reduce the size of responses, downloads, and cached pages), optimizing browser rendering (tweak browser layout of a page), and optimizing for mobile (tuning for characteristics of mobile networks and devices).
According to Google:
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.