Speed Index (SI) is one of the Web Vitals but not a part of Core Web Vitals. It meaures the user-perceived page load speed. Let's learn more about SI, how you can measure it and the steps to improve Speed Index scores.
What is Speed Index?
Speed Index (SI) is a performance metric that quantifies how quickly content is visually displayed during the loading of a webpage.
Unlike metrics that measure when certain parts of a webpage are loaded, SI focuses on the user's visual experience.
It calculates the average time at which visible parts of the page are displayed, making it an essential factor in assessing user-perceived page load speed.
It's important to note that SI is not a measure of when a page is loaded, but rather how quickly the user perceives the page to be loaded.
Is Speed Index an Important Metric?
Absolutely! SI is a crucial metric because it reflects the user's experience. A lower SI score indicates a better user experience, as it means that the visible parts of a webpage are loading quickly.
This can lead to higher user engagement, retention, and conversions. In fact, Google considers SI as one of the key metrics in its Lighthouse performance score.
Rossignol improved their load time by 1.9 seconds and cut their Speed Index by a factor of 10, contributing to a 94% improvement in conversion rate when compared to the year prior.
How to Measure Speed Index
Speed Index can be measured using various tools, with Google's Lighthouse and PageSpeed Insights being the most popular ones. Speed Index can also be measured on other Speed Testing Tools like GTMetrix, SpeedVitals, and WebPageTest.
Lighthouse provides a comprehensive audit of a webpage, including the SI score. It simulates a page load in a controlled environment and measures the visual progression of the load.
WebPageTest measures SI by capturing a video of the page load and computing the visual progression between frames.
What is a Good Score
In terms of Speed Index, a score of 4.4 seconds or less is considered excellent. This means that the majority of the content of the webpage is displayed within one second.
However, achieving this score can be challenging and depends on various factors, including the complexity of the webpage, the server's response time, and the network conditions.
A score between 4.4 sec and 5.8 sec needs improvement, while anything above that is considered poor.
Radins improved their Speed Index by 51% for the desktop experience and saw a significant increase in conversion (+12%) and a drop in the bounce rate (-25%).
How To Improve Speed Index
Improving SI involves several strategies:
Optimize Images: Images often make up the majority of a webpage's size. Use appropriate formats (like WebP), compress images without losing quality, and leverage lazy loading, which only loads images when they're about to enter the viewport.
Minify and Compress Files: Minify CSS, JavaScript, and HTML files to remove unnecessary characters. Use GZIP compression for reducing file sizes, which can significantly speed up the download time.
Use a Content Delivery Network (CDN): A CDN can help deliver your content faster to users around the world by serving it from the server closest to them. This reduces the time it takes for data to travel from the server to the user's browser.
Preload Key Requests: Preloading important resources can speed up their delivery. This tells the browser to download key resources as soon as possible, reducing the time it takes for them to become available.
Optimize Fonts: Preload important fonts and ensure text remains visible during webfont load. This can prevent a flash of invisible text (FOIT) or a flash of unstyled text (FOUT), both of which can negatively impact the user experience and the SI score.
Conclusion
Speed Index is a critical performance metric that directly impacts user experience. By understanding what SI is and how to improve it, you can enhance your website's performance and user satisfaction.
Remember, the goal is not just to improve your SI score, but to provide a better, faster experience for your users. After all, a happy user is a returning user.