YSlow analyzes web pages and tells you why they’re slow based on the rules for high performance web sites. YSlow is a Firefox add-on integrated with the popular Firebug web development tool. YSlow gives you:
* Performance report card
* HTTP/HTML summary
* List of components in the page
* Tools including JSLint

Performance View
YSlow analyzes any web page and generates a grade for each rule and an overall grade. If a page can be improved, YSlow lists the specific changes to be made.

Stats View
YSlow calculates the total size of the web page for both empty cache and primed cache scenarios, as well as information about cookies.

Components View
YSlow lists all the components in the page including their type, URL, Expires date, gzip status, load time, size, and ETag. You can also view the HTTP response headers for any component.

* Add an Expires header
Adding an Expires header to your components with a date in the future makes them cacheable, reducing the load time of your pages. Certainly this should be done with images, but that’s fairly typical. Go a step further and add it to scripts and stylesheets, too. This won’t affect performance the first time users hit your page, but on subsequent page views it could reduce response times by 50% or more.

* Gzip componentes
Gzipping all the js and css files types to reduce page weight. The greatest advantage is compressed to reduce the amount of data transmission network, thereby improving the client browser access speed. Of course, but also increase the burden on the server a little bit. Gzip is a more common form of HTTP compression algorithm.

* Configure ETags
1. Client requests a page (A).
2. The server Back to A, and A to add a ETag.
3. Client display the page, and pages together with the ETag cache.
4. Customers once again request pages A, and the last request to the server to return to the ETag transmitted to the server.
5. Server checks the ETag, and to determine the page since the last client request has not been modified, to return to direct response to a 304 (Not Modified) and a response of the air. So. ETag is a Tag to your components to see if they has been modified. If not here will be no request happened. Configure it is a good way.