Everything you should know about Core Web Vitals

By Sam Taylor • 25 March 2021 • 7 minutes reading time

With the Page Experience update looming, we’ve put together a guide for publishers on everything you need to know about Web Vitals and Core Web Vitals to prepare.

What are Core Web Vitals?

Core Web Vitals are a subset of Web Vitals, an initiative by Google to provide guidance on quality signals essential for a great web user experience. They apply to all pages and should be measured by site owners, each representing a facet of the user’s experience.

The current set for 2021 focuses on three elements: loading, interactivity, and visual stability.

It includes the following metrics:

  • Largest Contentful Paint (LCP): measures loading performance. To provide a good user experience, LCP should occur within 2.5 seconds of when the page first starts loading.
  • First Input Delay (FID): measures interactivity. To provide a good user experience, pages should have an FID of less than 100 milliseconds.
  • Cumulative Layout Shift (CLS): measures visual stability. To provide a good user experience, pages should maintain a CLS of less than 0.1.

Metrics relating to Core Web Vitals - Evolved Search

For each of the above metrics, a good threshold to measure is the 75th percentile of page loads, segmented across mobile and desktop devices. Read more here.

Google plans to update page experience signals on an annual basis, so this will continue to evolve. We will see more Web Vitals added as search ranking signals in the future.

What other page experience signals does Google use?

Google is on a mission to improve web standards and set a benchmark for all publishers. There are hundreds of factors that Google takes into account when it comes to web page rankings, many of which are kept away from the public eye to prevent manipulation.

Core Web Vitals will be added to Google’s existing page experience factors, which include:

  • Mobile-Friendliness: There have been multiple updates made to Google’s algorithm that we know of aimed towards mobile-friendliness and usability, including the colloquially-known ‘Mobilegeddon’ in 2015 and mobile-first indexing, which took a few years to roll-out and has been the standard for all websites since September 2020.
  • Safe Browsing: Google aims to protect its web users with Safe Browsing, an initiative they created in 2007. It gives users the ability to protect themselves from malware, unwanted software, and social engineering.
  • HTTPS: Back in 2014, Google announced that HTTPS would be used as a ranking factor. It is rare to see a site not utilising HTTPS encryption nowadays, showing the influence that this change has had. Since July 2018, Chrome browsers have also shown a ‘not secure’ label when visiting sites to warn visitors of potential security risks.
  • No Intrusive Interstitials: In January 2017, Google released an update to their search ranking algorithm, focusing on sites with ‘intrusive interstitials‘. In this, pages where content is not easily accessible from mobile searches due to an intrusive pop-up or standalone interstitial may no longer rank as high.

Image shows signals used as part of Core Web Vitals - Evolved Search

When will this update be released?

In May 2020, Google first announced plans to incorporate more page experience signals into its search algorithm and announced the expected launch date in November 2020.

Currently, the page experience update that will focus on Core Web Vitals is planned for May 2021, though there is no specific date announced just yet.

As with previous pre-announced updates, it may be that Google will announce the release as it is rolling out, so you should expect it any day throughout the month unless otherwise informed.

Update 20/04/2021: Google have now postponed the update and plan to release it in mid-June 2021. They have said that the release will be gradual, to ensure that they’re able to catch and resolve any issues that may occur. We should not expect to see the update play its full role until at least the end of August, which gives publishers a lot more time to prepare.

What can I do to prepare for the update?

Google has provided us with several tools to audit and understand our page quality and experience.

You can use some, or all, of the following tools to gain an understanding of what your site needs to change to pass the Core Web Vitals assessment:

Search Console

Under ‘Enhancements’ in Google Search Console, you’ll find a report on Core Web Vitals – one for desktop and one for mobile. This uses the Chrome User Experience Report, which collects anonymised real user measurement data. Clicking ‘open report’ will provide you with a list of pages that do not pass Core Web Vital assessments and which elements you need to focus on.

Below is a screenshot from one of our clients, Ocean Finance. A recent blog migration to a new CMS with optimisations and tweaks to existing landing page JavaScript significantly improved aggregate CLS, FID, and LCP values.

Google PageSpeed Insights & Lighthouse

In late 2018, Google added Lighthouse – a tool that audits performance, accessibility, PWAs, SEO, and best practices – to PageSpeed Insights.

You can either use PageSpeed Insights or Lighthouse as a standalone (either in Chrome Dev Tools or as a browser extension) to understand how Google scores your site, and what you need to look at improving.

Image shows Page speed insights as part of Core Web Vitals - Evolved Search

Please note that Google may use Core Web Vitals data from noindexed pages in the update, so do consider these as part of your assessments.

How to diagnose site speed issues?

There are multiple tools that you can use to diagnose the specific issues with your site that are impacting LCP, FID & CLS. Some of the ones we frequently use include:

Google Chrome DevTools

Earlier this year, Google added Web Vitals to DevTools (built into the Chrome browser) to allow developers to easily troubleshoot and pinpoint issues with page experience.

These use the same traffic light system as other tools Google has provided, with red being the lowest-scoring and green being the highest.

Image shows Google Devtools - Evolved Search

Traffic Light System as part of Core Web Vitals - Evolved Search

Google PageSpeed Insights & Lighthouse

As mentioned, PageSpeed Insights and Lighthouse are good tools to understand where your site stands and benchmark performance, but they also provide some details on issues impacting page experience, too. Scoring can sometimes fluctuate depending on multiple factors, so we wouldn’t recommend you use your performance scores as absolute benchmarks, but they’re good to use relatively.

Script example - Core Web Vitals - Evolved Search

Common Core Web Vital Issues

Largest Contentful Paint

LCP measures when the largest content element in the viewport becomes visible, and you can learn more about how to optimise LCP here. The most common causes include:

  • Slow server response time
  • Render-blocking JavaScript and CSS
  • Slow resource load times
  • Client-side rendering

First Input Delay

To optimise First Input Delay, we would recommend reading Google Developer’s guide on how to optimise FID. Typically, the causes of poor FID are:

  • Long tasks
  • JavaScript bloat, heavy execution times and inefficient chunking.
  • Third-party scripts, like tags and analytics.
  • Blocked main thread

Cumulative Layout Shift

Google Developers have a good post on debugging layout shifts, listing these as the most common causes of CLS issues:

  • Changes to the position of a DOM element
  • Changes to the dimensions of a DOM element
  • Insertion or removal of a DOM element
  • Animations that trigger layout

Will I see a drop in rankings if my site doesn’t pass CWV?

As mentioned previously in this post, Google uses hundreds of signals to determine web page rankings, and Core Web Vitals will be just three of them. Looking back at a post from January 2017 regarding the ‘Intrusive Interstitials’ update, Google said:

“This new signal is just one of hundreds of signals that are used in ranking and the intent of the search query is still a very strong signal, so a page may still rank highly if it has great, relevant content.” – Google

Though posted four years ago, this is still relevant today. John Mueller, Search Advocate at Google, has echoed this sentiment a few times within the last year:

If you do not pass all of the requirements when the update is released, you shouldn’t worry too much – it may well be that your site still ranks well due to other factors Google is considering.

However, you should strive to achieve the best performance scores possible ahead of May and look to continuously improve your website as the web evolves for long-term success in SEO.

Update 20/04/2021: Google have postponed the release of the update and have reiterated the following: “As we have said before, while this update is designed to highlight pages that offer great user experiences, page experience remains one of many factors our systems take into account. Given this, sites generally should not expect drastic changes. In addition, because we’re doing this as a gradual rollout, we will be able to monitor for any unexpected or unintended issues.”

What other benefits are there to meeting CWV requirements?

Aside from the potential ranking boost that may come over time from meeting Core Web Vital requirements, there’s also the added bonus of enhanced user experience, which may ultimately lead to increased conversion across your site. Google released an impact calculator to estimate the potential revenue impact you could see by improving your mobile website speed – you can test your own site here.

Google will also be releasing a badge in search results to indicate sites that pass the Core Web Vitals thresholds.

This will not hold any ranking benefit, but it may influence click-through rate should it become a standard across all web users.

Clearly, our technical team has quite the handle on this stuff, so if you’re looking for a strategic search agency with a steady finger on the industry’s pulse, get in touch here.

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