Are you aware of how long it takes for your website to load on a visitor’s screen? This duration is measured by the Speed Index (SI), a performance metric influenced by everything above-the-fold.
The Speed Index (SI) plays a crucial role in your overall performance score on the Lighthouse report. A good Speed Index ensures a great user experience by quickly delivering content in the viewport.
To enhance your performance score, there are six metrics to optimize, with the Speed Index accounting for 10% of the overall grade computed by Lighthouse.
What constitutes a good Speed Index? How do you measure it? Which tools are the most reliable? Running a performance audit for your website can be overwhelming. Let us answer all your questions in this blog.
What’s the Speed Index in Lighthouse?
Speed Index is an important Lighthouse performance metric that shows how quickly the page content becomes visible to the user.
It measures the loading time of the visible part above the fold (in the viewport) and tracks the entire process of loading what users see before scrolling.
Lighthouse records a video of the loading process and calculates the differences between frames, using the Speedline Node.js module to calculate the final Speed Index score.
This metric represents the average rendering speed of a web page and its visual advancement, also known as visual completion. This metric is important for measuring loading time and directly influences how users perceive a website’s performance.
How is the Speed Index Measured by Lighthouse?
Web.Dev describes that this metric is a “comparison of the speed index of your page and the speed indices of real websites.”
You can calculate this metric with the help of a Lighthouse Scoring Calculator.
The latest version of Lighthouse (v10) computes six metrics to generate your final performance score. These include Core Web Vitals, Total Time to Interactive, Total Blocking Time, and the Speed Index:
Source: Chrome for Developers
This metric is crucial because it determines whether your page is perceived by search engine as fast or slow.
Speed Index (SI) = Interval time * (1 – visual completeness / 100)
Before diving into technical recommendations for improving your SI metric, let’s first define what constitutes a good SI score according to Lighthouse.
What’s a Good Speed Index Score?
A good Speed Index score time should be under 3.4 seconds.
Source: Web.dev
Speed Index vs. First Contentful Paint
If you’re familiar with the recent Core Web Vitals update, the Speed Index metric might seem similar to the KPI – First Contentful Paint (FCP). Let me describe the main difference and why both metrics are important.
First, Contentful Paint calculates the time it takes for the browser to render any content for the first time. In contrast, the Speed Index describes the rate at which the entire content above the fold appears.
Improving your FCP will also enhance your Speed Index, providing a better user experience by avoiding the dreaded white screen.
Which Tools to Use to Measure Speed Index?
To measure your Speed Index and overall performance score, you can use tools like Google PageSpeed Insights and GTmetrix, both of which are powered by Lighthouse technology.
– Using Google PageSpeed Insights (PSI) to measure the SI metric:
– Using GTmetrix to get the SI grade:
What Affects Speed Index?
The aspects related to a high SI score are listed in the diagnostics section of Google PageSpeed Insights.
Typically, four main issues contribute to a poor SI score:
– JavaScript code and unused JS
– JavaScript execution time
– Main thread work
– Third-party code
The “opportunities” and “diagnostics” sections generated by Lighthouse provide insights into how you can improve your SI and overall performance grade. The report assesses the impact of each error on your performance. Resolving these issues will significantly decrease your Speed Index grade and enhance the overall user experience of your WordPress site.
10 Ways to Maintain a Great Speed Index
1. Optimize Images:
• Use image compression tools to reduce file size while maintaining visual quality.
• Choose appropriate image formats (e.g., JPEG, PNG, WebP) based on the type of content.
• Consider lazy loading images to load them only when they enter the viewport, shortening initial page load time.
Also Read: Master the Art of Image Optimization
2. Minimize Render-Blocking Resources:
• Identify and prioritize critical resources needed for initial page rendering, such as JavaScript and CSS files.
• Minimize the implementation of inline CSS and JavaScript and externalize them into separate files.
• Use asynchronous loading for non-essential resources to prevent them from blocking page rendering.
3. Reduce JavaScript Execution Time:
• Minimize the size and complexity of JavaScript code by removing unused or redundant code.
• Optimize JavaScript performance by using techniques like code splitting, lazy loading, and bundling.
• Consider using modern JavaScript frameworks or libraries that prioritize performance, such as React or Vue.js.
4. Utilize Browser Caching:
• Set appropriate caching headers to instruct browsers to cache static resources like images, CSS, and JavaScript files.
• Implement versioning or cache busting techniques to ensure that updated resources are fetched when necessary.
• Leverage browser caching to reduce server requests and improve page loading times for returning visitors.
5. Implement Lazy Loading:
• Load images, videos, and other resources asynchronously as the user scrolls down the page.
• Prioritize loading content above the fold first to improve perceived performance.
• Use native browser lazy loading attributes or JavaScript-based lazy loading libraries to implement lazy loading effectively.
6. Prioritize Above-the-Fold Content:
• Refine the critical rendering path to ensure that above-the-fold content is delivered and rendered quickly.
• Minimize the use of large or unnecessary elements above the fold that could delay initial page rendering.
• Inline critical CSS or use inline styles to style above-the-fold content and avoid render-blocking CSS files.
7. Minimize Third-Party Scripts:
• Evaluate the necessity of third-party scripts and consider removing or replacing them with lighter alternatives.
• Load third-party scripts asynchronously to prevent them from blocking page rendering.
• Use content security policies (CSP) to control and restrict the execution of third-party scripts on your website.
8. Enable Gzip Compression:
• Configure server-side gzip compression to reduce the size of text-based resources like HTML, CSS, and JavaScript files.
• Enable HTTP compression using server configurations or web server modules to compress responses before transmitting them to clients.
• Monitor server performance to ensure that gzip compression does not negatively impact server resources or response times.
9. Utilize Content Delivery Networks (CDNs):
• Distribute static resources across multiple servers located in different geographic regions to lessen latency and improve loading times.
• Select a reputable CDN provider that offers edge caching, global network coverage, and efficient content delivery.
• Configure caching policies and optimize cache settings to ensure that CDN resources are served quickly and efficiently.
10. Regularly Monitor and Optimize:
• Use performance monitoring tools like Google Lighthouse, GTmetrix, or WebPageTest to assess website performance and identify areas for improvement.
• Monitor key performance metrics such as Speed Index, First Contentful Paint, and Total Blocking Time to track progress over time.
• Continuously optimize and fine-tune website performance based on real-time data and user feedback to maintain a fast and responsive user experience.
Final Thoughts
Improving the Speed Index metric can lead to a significant enhancement in the perceived speed of your content’s appearance above the fold for visitors.
While the Speed Index reflects the visual completeness of the page, it does not discern between critical and non-critical content for the user. As a result, it should not be considered a substitute for other Lighthouse metrics.
FAQs
Can a poor Speed Index affect my website’s SEO?
Yes, a poor Speed Index can negatively impact your website’s SEO. Search engines like Google consider page speed to be a ranking factor, and a higher Speed Index can lead to lower rankings and reduced visibility in search results.
Is Speed Index relevant for both desktop and mobile sites?
Yes, the Speed Index is relevant for both desktop and mobile sites. However, mobile performance is particularly critical due to varying network conditions and device capabilities, making it essential to optimize for a fast Speed Index on mobile.
How often should I monitor my Speed Index?
It’s advisable to monitor your Speed Index routinely, especially after making mandatory changes to your website, such as redesigns, adding new features, or updating content. Continuous monitoring helps ensure that your page speed remains optimal.
