Web Performance Calendar

The speed geek's favorite time of year
2011 Edition

All about YSlow

by Betty Tso

Betty leads the Exceptional Performance Engineering team at Yahoo and drives the engineering execution & development for Yahoo!'s top Web Performance products like YSlow and Roundtrip.

Betty is an Evangelist on Web Performance Optimization best practices in multiple Yahoo! offices; speaker at conferences, Summits and top notch universities. Betty is also the Co-president of Yahoo! Women-in-Tech organization.

Since 2007, millions of developers have been using YSlow to help them find out ways to make their web pages load faster. YSlow score has been the standard for Performance measurement in dev, QA and production stages.

YSlow first started as a bookmarklet by Steve Souders while at Yahoo!, and soon became a popular Firefox extension. Over the past year, Marcel Duran built YSlow Chrome extension, Opera extension and Safari extension. In order to also support Mobile device as well as other browsers, YSlow was also made available as a bookmarket in June 2011 with fresh shinny code and new architecture.

While speaking at Velocity China on December 7, 2011, our team announced the release of YSlow for Command Line beta, with courtesy to our FE tech lead, Marcel. This version leverages Node.js and takes .har files as input to generate YSlow score for an URL. Several output options are available – JSON, XML and plain text. Users can also pipe the result to a beacon server, such as http://www.showslow.com/beacon/yslow/ and view the result in a graphical UI. For complete YSlow beacon spec, refer to users’ guide on getyslow.com.

The diagram below captures the timeline of YSlow development over the past few years (click for a bigger image):

Do you know?

  • YSlow can also be used as a framework to build extensions that talk to browser, refer to Stoyan Stefanov’s article for code samples: Web Testing Framework
  • Starting from v3.0.5, YSlow has a new feature – one-click-add-cdn to CDN custom list, which allows user to add CDNs to a custom list when applicable
  • YSlow’s social feature lets user share their YSlow score with Facebook and Twitter friends; the links shared points to YSlow Scoremeter on getyslow.com . With the Scoremeter, user is able to estimate the impact of a fix on the resulted YSlow score. Click here for a sample link shared on my Facebook: example Scoremeter
  • YSlow support for Jdrop will be released next, which provides a way to pipe YSlow result in JSON format and store them on the cloud for further analysis. This is especially useful for user running YSlow for mobile screens.

As always, we would love to hear your feedback. Please contact us in our Exceptional Performance Yahoo! group. You can also connect with us on Facebook and Twitter.

Special thanks to Lauren Tsung, who created the infographic in this post. Lauren is currently working as an interactive designer in Yahoo! System Tools team.