YSlow
More info
- Creator
- Steve Souders / Yahoo
- Released
- Type
- Performance tool
- License
- BSD
Yahoo's 2007 browser add-on that graded page performance A–F and turned front-end optimization into a discipline of its own.
YSlow was built by Steve Souders at Yahoo and released in 2007 as an extension to Firebug in Firefox. The tool analyzed a loaded page against Yahoo's "Best Practices for Speeding Up Your Web Site" — fewer HTTP requests, compression, cache headers, a CDN — and gave an overall grade from A to F.
The idea of measuring and grading front-end performance was new; optimization had mostly meant the server until then. YSlow and Souders' book popularized rules that still hold and paved the way for Google PageSpeed, Lighthouse and Core Web Vitals. The tool was open-sourced in 2011 but faded along with Firebug. Its legacy is the concept rather than the code: that a page's speed can be measured and improved systematically.