Illustration of our web developers measuring and optimizing core web vitals metrics LCP, CLS and FID
By  Rank Grow Digital

Core web vitals are a group of metrics defined by Google intended to be used as signals to determine your website’s overall performance for your business. Largest Contentful Paint, First Input Delay, and Cumulative Layout Shift are the three most essential for optimal website performance. By keeping the scores of these metrics in compliance, your visitors will have the best user experience with minimal bounce rate, leaving your SEO unaffected.

Since overall performance and mobile performance are key ranking factors in search engines, complying with these metrics will help you avoid any SEO-damaging penalties. 

Google has stated that their new page experience signals, combined with core web vitals and their existing search signals, will roll out in May 2021 and will be essential for ranking. This means that all websites in compliance will have a chance to rank above those that are not.

The following core web vitals break down into several metrics intended to measure loading performance, interactivity, and visual stability, all of which are critical for an excellent user experience.

Developer optimizing LCP largest contentful paint metric representing web page loading performance.

Largest Contentful Paint (LCP)

Largest Contentful Paint, or LCP, is a metric that is part of Google’s core web vitals group. It defines the essential signal indicators of your website’s performance, which contribute to your ranking. LCP as a metric is used to determine how long it takes the page’s largest element to load. While other metrics aim to measure similar aspects of your sites’ pages, they can be complicated or even misleading in estimating the correct load time. LCP tends to follow the KISS principle in its simplicity as a metric, giving you the most accurate measurement.

LCP will focus on basic content that appears within the screen’s viewing area, such as paragraphs, headings, images, videos, plus anything with a background image. However, as the name suggests, it will pick the largest web page element to measure load time and represent the page in its entirety. In some cases, the detection of this could be different between tests. However, it should average out to stay accurate with a large enough sample for analytics.

A typical example of LCP may be a “hero” slider, banner, or a featured image in a new webpage or blog page. Additionally, advertisements can also be considered LCP and could cause a negative impact. Since LCP targets the largest element it finds, you will want to ensure that you have your website assets optimized and loaded in the best possible manner.

A high LCP score, caused by the elements that take up the most space on your screen to load slowly, will leave visitors with a bad impression. It can make the site “feel” like it takes longer to load. That user perception could mean higher bounce rates and fewer conversions, affecting your business and SEO to capture more incoming leads. Load time is even more critical on mobile due to its growing use and the fact it has smaller resources to access the same content. Given this, the metrics are even stricter on mobile than desktop. With both mobile and core web vitals being a ranking factor, this means keeping a close eye on LCP and other metrics in general.

How to improve Largest Contentful Paint (LCP) with optimization.

The target score for LCP is 2.5 seconds or less, which can be accurately measured using the 75th percentile of page loads, divided between mobile and desktop. The same threshold can be used to determine your FID and CLS scores as well.

Developer optimizing FID First Input Delay metric which represents the interactivity of a webpage.

First Input Delay (FID)

First Input Delay or FID is a metric intended to measure interactivity performance. It measures the time it takes to get a response from your very first action on a web page. If the response to your action takes too long, this can irritate your visitors, so keeping your FID score at 100 MS or less will keep you in compliance.

How to improve First Input Delay (FID) and optimize to increase your website performance.

To give you an idea of the effects of a bad FID score, imagine visiting a poorly structured website. You try to sign up for the newsletter or add an item to the cart but get a delayed response. The FID on that page is high, most likely causing you to get frustrated and leave. Keep in mind this metric measures the 1st action you take, not just any action. Your server could be loading the whole site slowly, or the site’s essential actions may be poorly coded. Either way, you’ll feel a slow response or even wonder if maybe your computer is the problem, and that is not good for business!

So the end result for your business would mean not capturing a lead or even a lost sale (or potential long-time customer). This, as a domino effect, would also create a bounce, increasing the overall bounce rate and impacting your SEO and future leads.

Optimization of CLS Cumulative Layout Shift metric, improving the visual stability of a webpage.

Cumulative Layout Shift (CLS)

Cumulative Layout Shift or CLS is the metric that ensures your site is reliable and your visitors have a great user experience by providing visual stability. By reliable, we refer to the fact that the site does not move things around randomly or unexpectedly. This can also include bad UX decisions that will frustrate or upset your users.

Having layout shifts can mean that while a page loads, something significantly moves around, affecting you. An example of this can be having text change from big to small or small to big. You could have text or an image jump from one place to another unexpectedly, causing you to lose your place while reading. You could have a popup that shows before you click a button and cause you to follow a link unexpectedly. This shift in content could lead to user errors, which can discourage visitors.

CLS can also slow down the site because anytime you have a significant change on the page, the browser has to re-render what you see, which takes resources. Things moving around on a webpage without being prompted could cause users to view your site as annoying, “buggy,” slow, or get upset and not want to use your site at all.

How to improve Contentful Layout Shift (CLS) core web vital with optimization.

Like all other core web vitals, a bad CLS score or high CLS can have adverse effects on your SEO and should be monitored and kept in compliance, especially mobile. A healthy CLS score should be at 0.1 or less, which ensures stability. Anything higher will need improvement to stay in compliance and keep your ranking up.


Other metrics can affect user experiences, such as First Contentful Paint (FCP), Total Blocking Time (TBT), Time to Interactive (TTI), and your Speed Index (SI). However, your LCP, FID, and CLS are the essential metrics to focus on to ensure your site’s best user experience and SEO performance.

Need help getting your website(s) into compliance? Our team can guide you and implement the solutions you require to get positive results for your business. 

Our Own Results Core Web Vitals in compliance 100% desktop & mobile.

Take a look at our Google PageSpeed results, and see what we can do for you. If you’d like to know how your website performs, please contact us for a free analysis. We are ready to help your business GROW!

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