description YSlow Overview
YSlow is an open-source tool that analyzes website performance by measuring First Contentful Paint, Largest Contentful Paint, and Cumulative Layout Shift, providing data to identify and address potential bottlenecks impacting page load speed and user experience.
help YSlow FAQ
What is YSlow?
YSlow is an open-source website performance analysis tool originally associated with Yahoo. It evaluates pages against performance rules and reports possible causes of slow loading.
Does YSlow measure Core Web Vitals such as LCP and CLS?
YSlow's original rule-based analysis is not the same as Google's Core Web Vitals measurement. For current LCP, INP, and CLS reporting, tools such as Lighthouse or Chrome's performance tools are more appropriate.
What does YSlow look for on a webpage?
It checks issues such as excessive HTTP requests, inefficient caching, large assets, and poorly optimized page structure. The result is meant to guide practical performance fixes.
Who created YSlow?
YSlow was created at Yahoo as an open-source tool for web performance analysis. It became known for turning a page-speed audit into a set of named recommendations and a score.
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