#Core Web Vitals

Google Core Web Vitals: what happened and why should you care?

by Rémon Bloemberg 11 minute read
Google Core Web Vitals: what happened and why should you care?

The Google Core Web Vitals… It’s the talk of the town in the SEO community. All for good reasons. Improving them makes your website faster and provides a better user experience. But there is more besides conversion optimization and lower bounce rates: they are ranking factors for Google Search now. In order to “pass” the Core Web Vitals assessment, you need to score +75% on all three Core Web Vitals.


In this article, we'll give you a better understanding on how the Google Core Web Vitals work. We explain why you should care to improve the performance of your website for your users, but also from a business perspective. The best reason to create a better performing website is that visitors love fast sites that are easy to use, on any device, from any location. And for a online business perspective it’s very simple:

You’ll make more money if you’re providing a great user experience.


Since its beginnings, Google has made regular (almost daily) changes to its search algorithm. And, every few months, things get even more tense for marketers and search experts when Google rolls-out its Core Updates.

It has become what we expect from Google and its quest to deliver the most reliable and relevant information to serve user queries. However, recently Google increased importance on user experience, so it’s not just a matter of delivering quality information but doing so in a user friendly format and manner. Therefore, Google recognizes a number of variables used to measure user experience. And, inattention to these variables and providing a subpar user experience may influence organic traffic and rankings as from mid 2021.

Google has informed us that this new focus on Core Web Vitals will be underway by the end of August 2021. If your site has recently been experiencing a rise (or fall) in web traffic and rankings, it’s likely due in part to your attention (or inattention) to user experience. Read on to learn more about Core Web Vitals and what you can do to keep your site optimized.

The (Core) Web Vitals as a way of measuring user experience objectively

Speed and performance is relative and highly depends on one’s web connection and device. Two websites can finish loading in the same time yet one of them can be perceived as the quicker one due to progressive loading instead of displaying everything when fully loaded. A website can also appear to load quickly while it responds slowly. When measuring performance it’s necessary to use objective and quantitative metrics. In order to make sure that the used metrics are useful to the user, Google asks four different questions:

  • Is it happening? Did the navigation start successfully? Has the server responded?
  • Is it useful? Has enough content rendered that users can engage with it?
  • Is it usable? Can users interact with the page?
  • Is it delightful? Are the interactions smooth and natural, free of lag and jank?

In order to answer the questions, Google identified several Web Vitals that can help any website owner, marketer or developer to quantify website experience and identify improvement opportunities. For quite some years they provided different tools to analyze and report on performance. However this might’ve been a bit too much to handle and they wanted to simplify the Web Vitals system.

In the second quarter of 2020, Google introduced some essential metrics which helps websites to focus on what matters most: the Core Web Vitals. The Core Web Vitals are a subset of Web Vitals that apply to all website pages and should be monitored in order to keep your website healthy. Every Core Web Vital represents a certain part of user experience and is measurable both in a lab-setting where page load is simulated or in the field with real users in order to show real user experience (we’ll get back to that later in this blog).

The Core Web Vitals apply to all web pages and relate to user experience. These aspects focus on load speed, interactivity, and visual stability. However, Google mentions these criteria will evolve with time.

What are the Core Web Vitals variables?

The 2020 set of Core Web Vitals includes three metrics that act as search signals for page experience and will be used to determine your website ranking: the Largest Contentful Paint for loading speed, First Input Delay for interactivity and Cumulative Layout Shift for visual stability.


Largest Contentful Paint (LCP) measures the loading performance of your largest text block or image element. It aims to measure when the page’s main content elements have finished loading. In order to provide a good user experience, LCP should occur within 2.5 seconds of when the page first starts loading. You should improve if you score between 2.5 and 4 seconds. After 4 seconds you won’t pass the LCP metric.

The most common causes of a poor LCP score are:

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


First Input Delay (FID) measures interactivity of your website. It’s the speed and responsiveness of the first interaction when visiting the website for the first time. Anything less than 100 milliseconds is considered good. Between 100 and 300ms needs improvement and anything above will give you a bad score.

The most common causes of a poor FID score are:

  • Heavy JavaScript or CSS execution before rendering the HTML
  • Disordered coding
  • Images and scripts loading in a non-orderly matter


Cumulative Layout Shift (CLS) measures visual stability. CLS helps quantify how often users experience unexpected layout shifts, which is rather annoying if you’re reading but can also cause misclicks. To maintain a good user experience, your website should score less than 0.1. Between 0.1 and 0.25 needs improvement and your layout shifts too much if you score above 0.25.

The most common causes of a poor CLS score are:

  • Unspecified image dimensions
  • Dynamic advertisements
  • Heavy embeds and iframes without dimensions
  • Web fonts causing FOIT/FOUT
  • Actions waiting for a network response before updating DOM


Testing your (Core) Web Vitals performance

Google developed a nice tool to measure the Core Web Vitals performance of your website, and we put a simplified version on the homepage of our website so you can test it yourself.

You didn’t pass the test? This could result in a number of things. You may experience a higher bounce rate and a drop in organic traffic. We’ll provide a detailed analysis on your site and/or install a tool to monitor ongoing performance. We follow-up with expert suggestions that we pass along to your developer or initiate for you. The ongoing health of your site is ensured through constant analysis and monthly reporting.

Did you pass the test? That’s great! But, it doesn’t mean your site is safe from future updates. Even if you did pass, having us provide you with an analysis allows for opportunity to improve your bounce rate, improve click throughs, and increase on-page time. In short, we improve your site for increased traffic and sales.

We recommend you to test if you pass the Core Web Vitals via our website or the free tool that Google offers. When you have performance problems, you can no longer ignore them because sooner or later this will influence your business performance. Often, problems are easy to address. Analyzing your site is not a one-time process but an ongoing strategy, ensuring you stay ahead of the competition and your website remains a lucrative asset.

How does Google determine the Core Web Vitals scores?

Performance metrics are in general measured in two different ways:

In a lab setting with tools that simulate a page load in a consistent and controlled manner. Lab data is gathered in a controlled environment with a predefined set of network and device settings. The idea behind a lab test is to control for as many factors as you can, so the results are (as much as possible) consistent and reproducible from run to run.

In the field with real user data about actual page loading and interaction so it reflects the actual devices, network conditions and geo-location of your users. The data is gathered by monitoring all users who visit a page and measuring through the Chrome User Experience Report (crUX). The field testing is also known as real user monitoring (RUM).

Google uses both lab and field data depending on the availability of field data because a website obviously should have enough visitors in order to measure. However since most websites have enough real user data we will explain the scoring system based on field data.

It’s good to know that field data isn’t just one score but rather a distribution of numbers. This is because every website visit will lead to different results depending on the external conditions. For some people your site might be slow while it is quick for others. But even the user can influence the performance itself, for instance from switching from 4G to a WiFi connection on the same device.

So, if you look at the numbers over a period of time you will always see very low and high scores and when a tool gives a single metric, it will basically represent a specific point in the distribution. The Core Web Vitals tool uses the 75th percentile value in order to determine if you pass the test. This method ensures that the majority of visits experience that certain level.

Let us look at an example! In case of Amazon.com we see the following score on Largest Contentful Paint (LCP):


This means that:

  • 87% of visits saw an LCP of 2.5 seconds or less (good).
  • 7% of visits saw an LCP between 2.5 and 4 seconds (needs improvement).
  • 6% of visits saw an LCP greater than 4 seconds (poor).

The conclusion here is that they pass the test on LCP.

If you ran the test on our website you also noticed that there is a fourth non-Core Web Vital present, the First Contentful Paint. Apparently, Google wants to highlight this variable so it could be possible that this one eventually will be a fourth Core Web Vital, but that is just speculating for now. We do advise you to keep track on this metric, but also on some other non-Core Web Vitals which will tell you more about in the next paragraph.

The non-Core Web Vitals

Next to the Core Web Vitals, Google also identified several non-Core Web Vitals. Since they are not seen as one of the major Web Vitals factors, should we care about them?

A recent Core Update increased the call to urgency. It’s not a choice but an obligation to care about user experience. But long before it was mandatory, it was already highly advisable to keep track of the Page Experience if your website is essential in your business model and users are important to you.

Keeping your website healthy has a direct relation with your business performance: happy users stay longer on your website, they convert more and they come back more often. So at Iron/Out this is what we preach: no matter if Google takes the Web Vitals into account or not: the better your website is for your user, the better this is for your business. Therefore we advise every website owner to include the non-Core Web Vitals in your audits.

Website health directly relates to business performance. Iron/Out celebrates Google’s Core Web Vitals because such analysis echoes our commitment to improving user experience. Therefore, we strongly encourage clients to include no-Core Web Vitals when auditing a site.

So… what are the current non-Core Web Vitals?

  • First Contentful Paint (FCP) measures the time from when a page starts loading to when anything of that page’s content is rendered on the screen. A fast FCP reassures your users that something is happening and that is why we see this metric also as the fourth variable in Google’s PageSpeed Insights test. Anything below 1.8 seconds is considered good, between 1.8 and 3s needs improvements and above 3s is poor. If you test your website (we get to that later) you’ll also notice that the FCP is present in the standard report. Apparently, Google wants to highlight this variable so it could be possible that this one eventually will be a fourth Core Web Vital, but that is just speculating for now.
  • Time to Interactive (TTI) measures the time from when a page starts loading until it’s fully interactive. You’ll see that you have slightly more time than the Largest Contentful Paint variable since a good TTI score is below 3.8s, between 3.8 and 7.3s needs improvement and slower than 7.3 means you’ll score poorly.
  • Total Blocking Time (TBT) measures the time in milliseconds between First Contentful Paint (FCP) and Time To Interactive (TTI). It shows how (un)responsive a page is before it comes fully interactive. Anything below 200ms is considered good. Between 200 and 600ms needs improvement and above 600ms will be considered as poor performance.
  • Speed Index (SI) measures how quickly the content of a page is visibly displayed while loading a page. In order to measure the SI, Google captures a video of the page loading in the browser and computes the visual progression frame for frame. Anything below 3.4s is considered good. You need improvement if you score between 3.4 and 5.8s. Below 5.8s you will score poorly.

Google currently only identified Largest Contentful Paint (LCP), First Input Delay (FID and Cumulative Layout Shift (CLS) as Core Web Vitals, however we do advise you to keep an eye on the non-Core Web Vitals as they might become more important. It’s also possible that other metrics will be added to measure the user experience more accurately, but Google promises only to make changes on an annual basis in order to keep up.

So why should you care about the Core Web Vitals?

Google’s recent Core Update increased the call to urgency for website performance optimization: it’s not a choice but an obligation to care about user experience. Something that’s always been intuitive, improving customer experience, is now also a vital process of every website owner. Those who already prioritize user experience and site speed may see some positive effects. However, there’s always room for improvement. And, many other businesses take risk in not paying greater attention to the Core Web Vitals.

At Iron/Out, we live by analysis and optimization. We don’t rest until every crevice of your site is operating at high performance and in position to wow your visitors. We’re relentless in pursuing and terminating web issues related to user experience, performance, and speed. If you want to know how your website is performing, you can do our free test. If you don’t pass the test, we advise you to do a performance audit which creates a first deeper understanding about the current performance status of your website. It creates a data baseline and is our guideline for first improvements.