To improve Speed Index, make the visible part of your page appear sooner: load the top content first, reduce render-blocking CSS and JavaScript, optimize the main image, speed up your server and cut work that blocks the browser. Speed Index measures how quickly a page fills in visually, and Lighthouse treats 3.4 seconds or less as good on mobile. Check yours with our free website speed test.
Key takeaways
- Speed Index is a lab metric. It is not one of the Core Web Vitals.
- It measures how quickly the visible part of the page is filled in during loading.
- Faster server response, lighter top content and fewer blocking files all lower it.
- It carries 10% of the Lighthouse performance score, so it helps, but LCP, TBT and CLS carry more.
What is Speed Index?
Speed Index is a Lighthouse metric that shows how quickly the content of a page is visibly displayed while it loads. Lighthouse records the page loading, compares the frames and works out how fast the screen fills in. A lower value is better.
What is a good Speed Index?
Lighthouse treats a Speed Index of 3.4 seconds or less as good on mobile. It is one of five metrics in the performance score, with a weight of 10%. The larger weights belong to Total Blocking Time (30%), LCP (25%) and CLS (25%). See what a good PageSpeed score is.
How do you improve Speed Index? 8 fixes
1. Speed up your server
A slow first response delays everything that appears afterward. See what a good TTFB is.
2. Reduce render-blocking CSS and JavaScript
Files in the page head hold back the first view. Defer what you can and reduce unused CSS. See how to eliminate render-blocking resources in WordPress.
3. Optimize the main image at the top
Compress and resize it, use WebP and do not lazy-load it. See how to optimize images for page speed.
4. Load critical CSS first
Inlining the styles needed for the top of the page lets it appear while the rest loads. Many optimization plugins can do this.
5. Cut third-party scripts
Ads, chat widgets and trackers compete with your content for the browser’s attention. Delay or remove what you do not need.
6. Reduce JavaScript work
Heavy scripts keep the browser busy and delay painting. Remove unused code and split large files.
7. Optimize fonts
Use fewer font families and weights, and set font-display to swap so text appears while fonts load.
8. Use caching and a CDN
Caching and a nearby server shorten loading time for every file.
How do you check Speed Index?
Run your page through our PageSpeed Insights walkthrough steps or our free page speed checker on mobile and find Speed Index in the metrics list. Test two or three times, because lab values vary.
Is Speed Index the same as LCP?
No. LCP measures when the largest element appears. Speed Index measures how quickly the whole visible area fills in. They often improve together. Read more in website performance metrics.
Frequently asked questions
Is Speed Index a Core Web Vital?
No. The Core Web Vitals are LCP, INP and CLS. Speed Index is a lab metric.
Does Speed Index affect SEO?
Not directly. Google uses real-user Core Web Vitals. Speed Index helps you debug loading problems.
Why is my Speed Index high on mobile?
The mobile test simulates a slower phone and network. See why mobile scores are lower than desktop.
What is a good Speed Index on desktop?
Desktop values are usually lower, but check the thresholds shown in your Lighthouse report.
How long does it take to improve Speed Index?
Simple fixes such as image compression and caching can take under an hour.
Conclusion
To improve Speed Index, speed up the server, reduce blocking files, lighten the top of the page and cut third-party scripts. Re-test after each change with our free website speed test, and follow our guide on how to improve website speed.
