Speed is a crucial aspect in providing a pleasant user experience to visitors of your site. It ensures that a visitor's limited attention span and time are spent on actual content and not wasted waiting for images and scripts to load. Studies have shown that users will not tolerate more than a 4 second load time. If your site fails to offer a quick response, your users will leave.
In terms of SEO, Google is hinting at using PageSpeed score in its ranking algorithm. As developers and marketers look to optimize their sites, page speed should be among the top optimizations to consider.
Two of the major speed analysis tools are Google's PageSpeed and Yahoo's YSlow. Both require Firefox and the Firebug add-on.
Here's a run down of each:
"When you profile a web page with PageSpeed, it evaluates the page's conformance to a number of different rules. These rules are general front-end best practices you can apply at any stage of web development. We provide documentation of each of the rules here, so whether or not you run the PageSpeed tool - maybe you're just developing a brand new site and aren't ready to test it, you can refer to these pages at any time. We give you specific tips and suggestions for how you can best implement the rules and incorporate them into your development process."
"YSlow grades web page based on one of three predefined ruleset or a user-defined ruleset. It offers suggestions for improving the page's performance, summarizes the page's components, displays statistics about the page, and provides tools for performance analysis, including Smush.it™ and JSLint."
What's the difference?
PageSpeed and YSlow generally offer the same service, however there are differences in their calculations. Each service analyzes a page using a set of rules that they believe are most relevant to page speed and performance. Most of the rules overlap or are very similar to each other, but in general your scores should be comparable.
The benefit of GTmetrix is that you can analyze your page using both services, giving you different perspectives on how to optimize your code.
On the side you'll find a complete list of best practices as defined by each service.