Speed Index: What is it?

Speed Index: What is it?

What is Speed Index?

The Speed Index is a calculated, rather than measured, synthetic time statistic. It’s calculated based on visual changes between frames taken during page loading.

The average time it takes for a page to transition from its initial state (blank screen) to a visually complete state is displayed by the Speed Index. The page can continue to load components and process data in the background without affecting the Speed Index if the user’s viewport remains unchanged.

Each collected frame is evaluated for visual completeness above-the-fold until all information is visible, or the page is visually complete.

Speed Index - Visual CompletenessSpeed Index - Visual Completeness

Speed Index metric determines how visually complete, by analyzing the load of your page frame-by-frame.

Speed Index is a good way to track the impact of other metrics like LCP and FCP on the visual completeness of your page.

What Speed Index measures

The Speed Index is a metric that measures how quickly content is visually displayed on a page when it loads. Lighthouse begins by confirming the browser’s page loading and estimating the visual progression between frames. The Speed Index is a scoring system that considers numerous different speed characteristics, rather than a timing metric. The lower your website’s speed index score, the faster it actually performs.

The speed index metric’s holistic nature gives you a holistic view of your website’s perceived performance; however, it isn’t always useful for diagnosing problems, which is why other metrics are still important.

Speed Index - RenderingSpeed Index - Rendering

Speed Index – Optimized and Unoptimized Rendering

How Lighthouse calculate your Speed Index score

Based on data from the HTTP Archive, your Speed Index score compares the speed index of your page to the speed indices of real websites.

The following table explains how to read your mobile Speed Index score:

Speed Index
(in seconds)
Color-coding
0–3.4Green (fast)
3.4–5.8Orange (moderate)
Over 5.8Red (slow)

How Can You Boost Your Overall Performance Score

It’s usually better to focus on improving your overall Performance score unless you have a specific reason for doing so.

Determine which improvements will add the most value to your page by looking at the Opportunities section of your Lighthouse report. The greater the significance of the opportunity, the greater the impact on your Performance score. The Lighthouse screenshot below, for example, shows that removing render-blocking resources will result in the greatest improvement:

Speed Index - Opportunities SectionSpeed Index - Opportunities Section

Speed Index – Opportunities Section

How To Improve Your Speed Index

While every action you take to increase page load performance will improve your Speed Index score, fixing any issues identified by these Diagnostic audits should have a particularly huge effect:

Use lazy loading images

 Lazy loading refers to content that loads as the user scrolls down the page. Because many users won’t scroll below the fold anyway, this technique saves bandwidth and is ideal if the webpage’s call to action is near the top. Lazy loading might significantly slow down your overall performance if done incorrectly.

Add placeholders

Placeholders do not speed up image loading times, but they do help users wait by indicating that an image is on its way. If you have a website with a lot of images, they’re a must-have. A placeholder can be an empty space or a block of solid color that matches the image’s dimensions.

Accept Images that are progressive

Progressive images allow images to be rendered in little chunks, starting with low-resolution approximations and progressing to a complete image. Depending on the image format, the setup is slightly different.

Eliminate unnecessary downloads

Users may become unsatisfied if resources are designed to engage them to slow down your entire operation. You should come up with a way to measure the effectiveness of any such resource before you add it.

Compress your data

Compressing the assets on your website is a simple and effective way to save space, reduce load time, and improve overall site speed. Compressing your data, whether using a tried-and-true method like Gzip or a newer compression algorithm like Brotli, is a must.

Create a caching hierarchy

Cache misses are passed up through the layers until a cache hit is found or the request is directed to the origin server. Cache hierarchies are appealing because they can boost performance. Some of your cache misses will be hits in your neighbors’ caches. This minimizes wide-area network capacity and speeds up downloads even further.

Optimize your images

Use srcset sizing attributes or resize images on your server so that the display size matches the original size. Invest in an automated image compression software if at all possible, so that your image assets are always optimized.

Optimize your fonts

Every font should be available in all formats, including WOFF2, WOFF, EOT, and TTF. Because EOT and TTF formats aren’t compressed by default, remember to use Gzip compression. Because fonts are static and rarely require updates, revalidation tokens can help you improve your caching strategy.

Speed Index - Passed AuditsSpeed Index - Passed Audits

Speed Index – Passed Audits Section

Speed Index Can Help You Achieve Your Website Speed Goals

Using the Speed Index metric to increase the speed with which your content shows above the fold can help your visitors notice a significant improvement. To get a sense of overall performance, you’ll still need to look at render start and document complete. The speed index provides you with a unique perspective on the user experience provided by your website.

The page’s visual completeness is indicated by the speed index, but it does not indicate whether or not the content was critical to the user. As a result, it should not be considered a substitute for other metrics, but rather a useful and much-needed addition to the list.

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