YSlow explains the speed of a web page
Summary
With YSlow, Yahoo provides measurable rules that assess browser requests, caching, compression and page elements together. YSlow was originally developed at Yahoo. The first version worked as an extension for Firebug.
Ideas
- YSlow checks real page elements instead of only the response time of the HTML document.
- Fewer HTTP requests particularly shorten connections with high latency.
- Cache headers avoid repeated transfers of unchanged files.
- Compression reduces CSS, JavaScript and other text resources.
- Component lists map every resource to its technical effect.
- Rule profiles adapt ratings to different types of page.
Insights
- Users experience a loading chain, not the speed of a server.
- Measurable rules turn performance into a repeatable development task.
- Optimisations need separate tests with an empty and a filled cache.
- Individual third-party resources can slow down the entire page.
Facts
- The tool evaluated HTTP headers and loaded components.
- The source code was later made publicly available.
Recommendations
- Set a budget for requests, data volume and loading time.
- Version static files before setting long cache times.
- Only remove rules with a documented technical reason.
References
Links to the original source and the Web Archive open in a new tab.