PageSpeed Service was turned off on August 3rd, 2015. Please see Turndown Information for PageSpeed Service.
PageSpeed Service improves web page latency by changing the resources on that web page to implement web performance best practices. Each change that PageSpeed Service makes corresponds to a rewriter. The rewriters are applied in a pre-defined order on any resource being served by the service.
Here is a complete list of PageSpeed Service rewriters. The rewriters marked as default are enabled by default. Default rewriters can be turned off and non-default rewriters can be turned on through PageSpeed configuration.