What is Speed Index?
Speed Index measures how quickly the content of a webpage is visually displayed during the page load. It’s a critical metric in the Lighthouse performance report, as it directly impacts user perception of speed and usability.
Lighthouse captures a video of the page loading and computes the visual progression between frames to generate the Speed Index score using the Speedline Node.js module.
Speed Index Scoring Guide
| Speed Index (seconds) | Performance Indicator | Color Code |
|---|---|---|
| 0–3.4 | Fast | Green |
| 3.4–5.8 | Moderate | Orange |
| Over 5.8 | Slow | Red |
How Lighthouse Evaluates Speed Index
- The Speed Index score compares your page’s performance with real-world websites using data from the HTTP Archive.
- A faster Speed Index score contributes positively to the overall Lighthouse Performance Score.
Also Read: Total Blocking Time (TBT) in Core Web Vitals Improvement Strategies
How to Improve Speed Index
Here are some effective strategies:
- Minimize Main Thread Work:
Reduce the time spent processing heavy tasks to ensure smooth rendering. - Optimize JavaScript Execution:
Defer or lazy-load non-critical JavaScript files to enhance loading speed. - Ensure Text Visibility During Webfont Load:
Use fallback fonts or preload key fonts to avoid rendering delays.
Example of Speed Index Improvement
Scenario: A page has a Speed Index of 6 seconds due to heavy JavaScript files and unoptimized web fonts.
Steps to Optimize:
- Compress JavaScript files and defer non-critical ones.
- Use a system font as a fallback until the custom font loads.
- Prioritize visible content using lazy loading for images.
Result: The Speed Index improved to 3.2 seconds, changing the score from Red (slow) to Green (fast).
Also Read: Interaction to Next Paint (INP) in Core Web Vitals
Why Speed Index Matters ?
Speed Index reflects how quickly users see content during a page load, making it essential for improving:
- User engagement
- Conversion rates
- Search engine rankings
Improving Your Overall Performance Score
While optimizing Speed Index helps, focusing on your overall Lighthouse Performance score is usually more impactful.
- Check the Opportunities Section in the Lighthouse report to identify the changes with the most significant benefits.
- Example: Eliminating render-blocking resources (CSS or JavaScript) can drastically enhance performance.
Speed Index Key Takeaways
To improve Speed Index:
- Minimize JavaScript and optimize web fonts.
- Focus on visible content delivery.
- Regularly audit your page with tools like Lighthouse or WebPageTest.
A better Speed Index ensures a fast, user-friendly website that keeps visitors engaged and satisfied.