Speed Index is one of six metrics tracked in the Performance section of the Lighthouse report. Each metric captures some aspect of page load speed.
Lighthouse displays Speed Index in seconds:
What Speed Index measures
Speed Index measures how quickly content is visually displayed during page load. Lighthouse first captures a video of the page loading in the browser and computes the visual progression between frames. Lighthouse then uses the Speedline Node.js module to generate the Speed Index score.
Speedline is based on the same principles as the original speed index introduced by WebpageTest.org, but it computes the visual progression between frames using the structural similarity (SSIM) index instead of the histogram distance.
How Lighthouse determines your Speed Index score
Your Speed Index score is a comparison of your page's speed index and the speed indices of real websites, based on data from the HTTP Archive.
This table shows how to interpret your Speed Index score:
|Color-coding||Speed Index score|
|Over 5.8||Red (slow)||0–49|
See the Lighthouse performance scoring post to learn how your page's overall performance score is calculated.
How to improve your Speed Index score
While anything you do to improve page load speed will improve your Speed Index score, addressing any issues discovered by these Diagnostic audits should have a particularly big impact:
- Minimize main thread work
- Ensure text remains visible during webfont load
How to improve your overall Performance score
Unless you have a specific reason for focusing on a particular metric, it's usually better to focus on improving your overall Performance score.
Use the Opportunities section of your Lighthouse report to determine which improvements will have the most value for your page. The more significant the opportunity, the greater the effect it will have on your Performance score. For example, the Lighthouse screenshot below shows that eliminating render-blocking resources will yield the biggest improvement:
See the Performance audits landing page to learn how to address the opportunities identified in your Lighthouse report.