This article describes how to use the YSlow browser extension to measure a website's performance.
YSlow is a web browser extension that analyzes a page and provides page speed ratings.
You can significantly improve your site's speed by addressing each of the following testing criteria that YSlow uses:
Make fewer HTTP requests: The fewer items a browser downloads, the faster a site loads. For example, you can combine multiple CSS files into one file to reduce the number of HTTP requests. Combining small images into a single “sprite” reduces the number of connections to the server, and speeds up delivery of the page as a whole. CSS minification that automatically combines CSS files is also very helpful.
Put CSS at top: Pages appear to load faster when CSS code is at the top of the HTML document. Doing so enables the browser to begin applying CSS rules immediately as elements appear on the page. When CSS is placed after the HTML tags that it applies to, the page may not appear as intended until the CSS rules have been parsed by the browser.
Reduce the number of DOM elements: The fewer HTML elements in a page (and thereby in the Document Object Model, or DOM), the faster the browser can parse it. Using lightweight templates that use the least amount of HTML to build a page can improve this. The larger a page is in HTML tags, the more processing time and RAM the browser must use to render the page.
Do not scale images in HTML: When a browser receives an image larger than the intended viewing size, the file size is larger than necessary and takes additional time to download. Instead, you should store images on the server at their intended viewing size.
For more information about YSlow, please visit http://yslow.org.