Since this blog post went live, Google has announced that it is postponing the roll-out of its Page Experience Update.

The algorithm update will now begin gradually in mid-June, but won't come into full effect until the end of August 2021.


Page experience has always been important for SEO (and Conversion Optimisation).

But as of May 2021*, it will become officially important, as Google rolls out its Page Experience algorithm update.

Since this was first talked about back in May 2020, the world of SEO has been in a heightened state of alert, in anticipation of what is expected to cause significant fluctuations in SERP rankings.

We even wrote several paragraphs about it in our 2021 SEO trends and predictions blog post at the start of the year.

What is Google's Page Experience Algorithm Update (May 2021)?

As we said, user experience has always been important.

But it’s also been overlooked, with other areas that promised bigger wins and faster gains favoured time and again.

However, Google has finally made it clear that websites that neglect user experience will find it more difficult to achieve good organic visibility.

In order to measure and establish a holistic picture of page experience, Google has created the Core Web Vitals metrics.

These, together with other existing user experience signals, will make up the so-called “page experience signal”.

What are the Google Core Web Vitals?

In short, the Core Web Vitals are 3 metrics that centre around page speed, interactivity, and visual stability.

Largest Contentful Paint (LCP) – The time it takes to render the main content that is visible within the viewport. Google recommends a maximum of 2.5 seconds.

First Input Delay (FID) – The time it takes from a user’s first interaction with the page to the time the browser is able to respond. Google recommends anything below 100 milliseconds.

Cumulative Layout Shift (CLS) – Quantities visual stability (layout shifts) to ensure smooth interactions with the page. A good CLS score is 0.1 or less.

Let’s have a closer look at these metrics and the typical issues that cause a negative impact on page performance.

Page Loading Speed - Largest Contentful Paint (LCP)

Largest Contentful Paint (LCP) measures page loading speed.

More specifically, it measures the render time of the largest content element visible within the viewport (a hero image, text element, background image).

There are many causes that lead to poor LCP, with the most frequently encountered being:

Slow server response times This could be due to backend infrastructure, slow API responses, suboptimal database queries, etc...

Render-blocking JavaScript and CSS Scripts and stylesheets that block the first paint of the page but are not critical for the page’s core functionality.

Slow Resource Load Times Typically caused by non-optimised images/videos that take up much of the loading time.

Client-side Rendering Code that takes time to be parsed, compiled, and executed which ultimately slows down the overall rendering process.

If your website’s pages have an LCP of more than Google’s recommended 2.5 seconds, you should investigate the areas described above.

User Interaction - First Input Delay (FID)

First Input Delay (FID) measures the time from when a user first interacts with the page to the time the browser responds to the interaction.

This is typically affected by large JavaScript bundles and long JS execution times, which cause delays in event processing and painting, leading to unresponsive pages.

Google says FID time should be less than 100 milliseconds to ensure an acceptable user experience.

Addressing long FID times typically involves loading code asynchronously, splitting code to deliver smaller JS payloads and threading with web workers to run scripts in background threads, removing the heavy load from the main thread.

Visual Stability - Cumulative Layout Shift (CLS)

Google created the CLS metric to quantify layout shifts and help web developers achieve layout stability.

Unexpected shifts of visual content can be disruptive and have a negative impact on user experience. The recommended measurement for this metric is 0.1 or less.

The CLS score is a product of the impact fraction (percentage the shifted element takes up on the viewport) and distance fraction (the distance the shifted element has moved).

Common causes of unexpected layout shifts are images without explicitly defined dimensions, ads and embeds without dimensions, dynamically injected content and web fonts causing FOUT (flash of unstyled text) or FOIT (flash of invisible text).

Overall, it is important to ensure the browser can allocate the correct amount of space in the document while such elements are loading.

Other Search Signals for User Experience

Of course, the Core Web Vital metrics are not the only page performance signals that will be taken into account in the new ranking algorithm update.

They will be combined with existing signals such as:

Mobile-Friendliness – Considering most users will most likely view a website using a mobile device, ensuring the website is functional and well performing on mobile devices is key to a solid SEO strategy.

Safe-Browsing – User safety should be a top priority for webmasters, so fixing security issues to prevent malicious attacks is essential.

HTTPS-Security – Hypertext Transport Protocol Security (HTTPS) allows secure connections between the webserver and the browser, ensuring user information cannot be stolen and data cannot be corrupted.

Webmasters need to ensure that the website holds a valid TLS SSL certificate.

Intrusive Interstitial Guidelines – Popup ads can be extremely unpleasant (especially for mobile users) and, as such, they can have a negative impact on the site’s page experience ranking signal. It is recommended to avoid their usage whenever possible.

How to Prepare Your Website for the May 2021 Page Experience Update

Google is known for being cagey with its algorithm updates.

But, in this case, it hasn’t left webmasters and SEOs in the dark about how to measure and address the several areas that impact page performance.

In fact, soon after the Core Web Vitals announcement, Google incorporated relevant measuring capabilities for different existing tools.

Google Search Console

The new Core Web Vitals report on Google Search Console provides real-world data (field data) to help diagnose issues across different pages.

Since this report is built using field data, it will only report on pages for which it has enough data in the Chrome User Experience Report (CrUX) dataset.

In addition, the search console also contains reports on mobile usability and website security.


Lighthouse is a website auditing tool that helps with the detection of issues affecting performance, accessibility, progressive web apps and SEO.

The latest version (v6.0) introduced the LCP and CLS and TBT (Total Blocking Time), which are lab implementations of Core Web Vitals.

These metrics are also factored in calculating the Lighthouse performance score.

Page Speed Insights

PageSpeed Insights is the most widely used tool for detecting issues related to page loading speed.

You simply enter the URL for inspection, and it spits out all the relevant metrics and areas for improvement.

The current version of PageSpeed Insights not only reports on real-world data, but it also uses Lighthouse 6.0 lab data to enrich the report.

Chrome UX Report

This report (also known as CrUX) is a public database of field data (data coming from real user monitoring, also known as RUM) for millions of websites.

This data is available from PageSpeed Insights (on a URL-level) and the Public Google BigQuery project, which can be easily visualised using Google’s Data Studio.

Chrome Dev Tools

A great place to troubleshoot and detect page experience issues is the swiss knife for web developers, the Chrome Dev Tools.

The new Experience section on the performance panel is incredibly useful when it comes to detecting layout shifts, as it helps you visualise where shifts are occurring.

Dev tools do not report on First Input Delay, however, it provides you with the Total Blocking Time (TBT) metric, which is strongly correlated with FID.

Web Vitals Chrome Extension

This Chrome extension helps you measure Core Web Vitals on desktop and it comes in extremely handy if you want to quickly check on page performance during the development of a page or get feedback on the fly.

Final Thoughts

The inclusion of the Core Web Vitals into Google’s search algorithm cements the importance of user experience in modern digital marketing.

If it wasn’t clear already, we strongly recommend that you use the information and tools listed above to understand problems areas of your website.

Once you understand the opportunities for improvement, it’s essential you take action as quickly as possible to ensure your website aligns with UX best practice and is braced for the impact of Google’s incoming update.