Page Speed Insights : Understanding the performance report

PageSpeed Insights

With the impending Page Experience Update for June 2021 announced by Google in November 2020, web performance optimization (WPO) is a topic that becomes even more important for SEO.

The loading speed of a site has always been an important ranking factor.
If SEO consultants dedicate a good part of their technical audits to analyze the performance of sites, it’s because there are several benefits.

The benefits of web performance

There are several advantages for both the user and the SEO that can justify investing in web performance optimization and improving your loading speed. Among them, we can name:

  • Better user experience
  • Decrease of the bounce rate
  • Increased conversion rate
  • Increase of the crawl speed (and therefore of the crawl budget)
  • Increase in the indexation rate
  • Better organic ranking in the end

The best speed Testing Tools

Many free tools exist today to measure the loading speed of a site. The most famous are :

Reading the Page Speed Insights report

The Page Speed Insights speed loading and Web Core Vitals report is a speed auditing tool developed by Google, and is one of the most used and most controversial. We will start by detailing the different parts of the report.

1. The Score

At the top of the report, a score summarizing the performance of the page is provided. This score is determined by running Lighthouse to collect and analyze lab data on the page.

The most important thing is not to obsess with this score, but rather to use it as an indicator of progress over time, as a compass to measure the impact of each action implemented in the WPO strategy.

2. Field Data

Field data will not always be available in all reports.

It comes from the Chrome Index Experience Report, and provides user experience metrics at the URL level known by Google’s crawlers. These are averages of data collected over the last 28 days on visitors to your site.

This data is the closest to reality and will help us understand how users interact with your site.

3. Lab Data

This data comes from Google’s servers and is a reflection of how Google interacts with your site.

Page Speed Insights crawls the page with Lighthouse, which simulates how a mobile device or computer loads the page. Performance statistics are then calculated for that page (“First Contentful Paint” and “Time To interactive,” for example), including Web Core Vitals.

Page speed Insights Web Core Vitals

4. Opportunities

Before looking at each opportunity in detail, it is better to take a step back and have a general view of things to move forward more quickly.

Indeed, some aspects can be addressed simultaneously. For example, your report may show optimizations such as “Deliver images in next generation formats” and “Encode images efficiently” separately. However, these things can be corrected at the same time.

The same logic applies to tips like “Remove unused CSS resources” and “Eliminate resources that block rendering”. It is very likely that unused CSS resources are blocking rendering.

In the event that one has these four opportunities, it would be appropriate to take a step back and work on the general aspects: images and CSS.

Understanding Page Speed Insights metrics

Several metrics are analyzed in the Page Speed Insights report that will give us insight into the health of a website’s speed.

  • First contentful paint (FCP): FCP represents the time that elapses between the moment you request a page and the moment the first piece of content appears on your browser screen.

  • Time to Interactive (TTI): TTI measures the time from the moment a page begins to load until the page is fully rendered and able to respond to user interactions.

  • Total blocking time (TBT): TBT measures the time during which a page is unable to respond to user interactions since it began displaying content. It is the time between the FCP and the TBT.

  • Speed Index: The speed index is the display speed. It is a key metric to measure the user experience. It informs the time until the display above the viewport of a page is complete.

Core Web Vitals Reminder

The Google Page Speed Insight report focuses a lot on the Web Core Vitals. Here we will describe them very simply. If you need more detailed explanations, check this article

  • Largest Contentful Paint (LCP): This metric represents the time needed to load the largest content of the page in the visible part of the screen, before the first scroll of the user. It must be less than 2.5 seconds to validate the test.

  • First Input Delay (FID): FID measures the response time between the moment the user first interacts with the page and the moment the page responds to the request. In other words, if the page loads, but you click or scroll and it doesn’t respond to your request, the FID is slow. The FID must be less than 100 ms to pass the test.

  • Cumulative Layout Shift (CLS): CLS measures the stability of the site. For example, if you want to click on a link and there is an unexpected shift that makes you click on the link above, it will negatively impact the CLS. For an optimized CLS on your pages, it should be less than 0.1.


Loading speed and web performance have always been a ranking factor for search engines. However, it is important to note that this is a secondary ranking factor, and that it is always a priority to solve the SEO fundamentals before focusing on improving Core Web Vitals or loading speed.

All of these metrics can be worked on to validate the Web Core Vitals without significantly changing the way your site works or looks.

We will need to find the cause of the load slowdown or shift in elements in order to make the correct adjustments. Do not hesitate to contact us for a dedicated Web Core Vitals audit!