Definition
Speed Index is a web performance metric that measures how quickly the visible content of a page is rendered in the viewport during loading. Expressed in milliseconds, it represents the average time at which visible pixels are displayed. A low Speed Index indicates that content appears quickly for the user. Unlike LCP which measures a single element, Speed Index captures the overall progressive rendering. Google Lighthouse uses Speed Index as a primary metric with a recommended threshold of 3.4 seconds maximum.
Key Points
- Measures progressive rendering speed of visible content in milliseconds
- Google Lighthouse threshold: below 3,400ms
- Complementary to LCP and FCP for a complete view of perceived performance
Practical Examples
Before/after optimization
After inlining critical CSS and deferring scripts, a product page's Speed Index drops from 5,200ms to 2,100ms, significantly improving perceived user experience.
Industry benchmark
Analyzing Speed Index of the top 10 results for 'link building platform' reveals an average of 3,800ms, giving LemmiLink a performance target.
Frequently Asked Questions
Use Google Lighthouse (in Chrome DevTools), PageSpeed Insights, or WebPageTest. These tools automatically calculate Speed Index by analyzing progressive screenshots during loading.
LCP measures when the largest visible element is rendered, while Speed Index measures the average speed at which all visible content appears.
Go Further with LemmiLink
Discover how LemmiLink can help you put these SEO concepts into practice.
Last updated: 2026-02-07