YSlow

Visit Website

Status note (checked 11 November 2026): YSlow appears inactive: the GitHub repository’s last push was in February 2023, and its headless mode depends on the discontinued PhantomJS.

Introduction

YSlow is an open-source tool, originally from Yahoo!, that analyses web pages and suggests ways to improve their performance based on Yahoo!’s rules for high-performance websites. Its site explains that Yahoo!’s Exceptional Performance team identified 34 rules affecting page performance, and YSlow tests the 23 that are testable, grading pages against predefined or custom rulesets.

YSlow was available as browser extensions (Firefox, Chrome, Opera, Safari), a bookmarklet, a command-line tool for HAR files, a PhantomJS command line and a Node.js server. The code is at marcelduran/yslow on GitHub under the BSD-3-Clause licence, with about 2,200 stars.

The project is inactive: the last push to the repository was in February 2023, and its headless command line depends on PhantomJS, which is itself no longer developed. Modern browsers and performance practices (HTTP/2, Core Web Vitals) have moved beyond YSlow’s rules.

CheckOps keeps YSlow for its historical importance; use Lighthouse or PageSpeed Insights for current audits.

Many of YSlow’s ideas live on: Lighthouse’s opportunities and diagnostics cover caching, compression, minification and request reduction, with modern metrics such as Largest Contentful Paint and Interaction to Next Paint.

Features

1. Rule-based grading: Grades pages against 23 testable Yahoo! rules.
2. Custom rulesets: Define your own ruleset.
3. Component summary: Statistics about page components and sizes.
4. Multiple flavours: Browser extensions, bookmarklet, HAR CLI and Node server.

Pros

1. Free and open source: BSD-3-Clause.
2. Historically influential: Its rules shaped modern performance practice.

Cons

1. Inactive: No repository activity since February 2023.
2. Outdated rules: Predates HTTP/2 and Core Web Vitals.
3. Old dependencies: Relies on PhantomJS for headless use.

Usage and Links (Example)

If you need YSlow’s grading for a legacy reason, analyse a HAR file with the Node.js command line. For current work, use Lighthouse (in Chrome DevTools or CLI) or PageSpeed Insights, which cover modern performance metrics.

Its rule list remains a useful teaching resource for performance basics.

Pricing

Free and open source under the BSD-3-Clause licence. There is no paid edition.

Recommended for?

• Readers researching the history of web performance
• Teams maintaining legacy reports based on YSlow grades

Conclusion

YSlow pioneered rule-based web performance analysis but is no longer maintained. Use Lighthouse or PageSpeed Insights for modern audits.