The master branch is the highly optimized end product. Each improvement step is in its branch. Look at the Pull Requests on github or checkout the project and go to the branch that interests you and do a git diff to the previous branch to see what was changed. i.e.

git checkout use-assetic
git diff origin/js-at-bottom

1. start: Not optimized at all
2. js-to-bottom: Move all javascript to the end of the page to let browser render the page before the javascript has been loaded
3. use-assetic: Use assetic to combine all javascript and all css files into one each. explain debug mode / prod mode and deployment. cdn.
4. sprites: Gather all layout image elements into one file and use css to show the correct part of that image.
5. assetic-yui-compress: Activate the yui compressor to minify css and javascript.
6. cache-rules: Define caching rules to allow some caching to happen. All pages vary on cookie to avoid problems with caching proxies. Use cache busting for css and javascript.
7. varnish: Set up the caching reverse proxy Varnish to cache things. Manual cache invalidation on changes.
8. varnish-esi: Use edge side includes with varnish for different caching scopes and lifetimes.