The Speed Index (SI) metric is one of the 6 metrics that influence your Core Web Vitals score, as reported by tools like Google PageSpeed Insights. In this guide, you will discover what SI is, how to measure it and how to improve it.
What Is Speed Index (SI)?
The Speed Index (SI) metric indicates how quickly the content of a page becomes visually available. Lighthouse first captures a video of the page loading in the browser and calculates visual progression between frames. Lighthouse then uses the Speedline Node.js module to generate the Speed Index score, based on the same principles used by webpagetest.org to calculate site speed.
How Is the SI Score Calculated?
The good SI value to aim for on desktop is 1,300ms or less; an average score (needs improvement) falls between 1,300ms and 1.6s; a poor score is above 2.3s.
For mobile, a score below 3.4s is considered good; above 3.4s is considered poor; anything in between needs improvement.
Speed Index contributes 10 points out of 100 to the Core Web Vitals (CWV) score, across all 6 metrics.
How to Measure SI?
SI can be measured in the lab or in the field — ideally in the field. To see your score, you can use one or more of the following tools:
Field tools:
- Google Search Console
- Google PageSpeed Insights (field scores available when data is sufficient)
- web-vitals JavaScript library
Lab tools:
- Lighthouse / Chrome DevTools
- Google PageSpeed Insights
- webpagetest.org
Note that no specific improvement opportunities for SI will be shown by these tools — so let’s explore together how to improve this metric.
How to Improve Your SI Score?
The SI metric improves in line with your overall site load speed, broadly speaking. By improving the other metrics rather than focusing on SI specifically, you optimize it by default. The improvements that will have the greatest impact on Speed Index are:
- Minimize main-thread work
- Reduce JavaScript execution time
- Improve TTFB (Time to First Byte)
Since I have already covered these optimizations in detail in the articles on improving Largest Contentful Paint (LCP) and improving Total Blocking Time (TBT), I won’t repeat them here. I invite you to read those articles for precise, step-by-step guidance.
By improving LCP and TBT together, you address 55 out of 100 score points — and those same optimizations will naturally improve the two other metrics (FCP & TTI) — except for CLS, which is its own separate topic.