But offloading things to CDN is actually one of the things it doesn't do. You might be thinking of the PageSpeed Service [1] which is a Google-hosted version.
Note that docs above are for mod_pagespeed (Apache), but all of the same filters are available in nginx port as well - it's the same C++ code under the hood. In a nutshell: HTML, CSS, JS, and image optimization (resizing, recompression, WebP, auto-spriting, etc...)