asper team
19 May 2023 15 min to read

Core Web Vitals Audit: Optimization and Testing in a Nutshell

The speed and stability of your website can make or break the user experience and, ultimately, your conversion rates.

Thankfully, Core Web Vitals, a set of metrics introduced by Google, serve as a concrete and quantifiable measure of user experience quality.

Auditing Core Web Vitals is crucial for anyone aiming to provide a superior user experience and enhance their site’s visibility in search engine results.

In this article, we will comprehensively explore these metrics and their optimization, and provide you with practical tips and tools for a successful Core Web Vitals audit.

What are the Core Web Vitals?

The term “Core Web Vitals” refers to specific factors that Google deems vital in a webpage’s overall user experience. They consist of three metrics: Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS).

Core Web Vitals measure a page’s perceived loading speed, interactivity, and visual stability, respectively. As the web continues to evolve, these metrics have gained attention, especially among those who understand that superior website performance is no longer just a nicety – it’s an imperative.

In the SEO community, opinions on the extent to which these Google-defined metrics act as a direct ranking factor are divided. Some SEO professionals believe that while these vitals are essential for a good user experience and do have an impact on rankings, they are just one of many metrics that Google uses in their complex ranking algorithms. On the other hand, others believe that these vitals have a more direct impact on SERPs.

In fact, according to a study by Deloitte, a mere 100-millisecond gain in page load time can drive up to an 8% improvement in conversion rates.

Regardless of this ongoing debate, it is widely accepted that Core Web Vitals indeed impact your website’s performance and user experience, influencing your site’s ranking indirectly. Even if CWVs are not a direct ranking factor, any impact on user experience can affect engagement metrics such as bounce rate and time on site, which are confirmed to be considered by Google in SERP rankings.

Therefore, optimizing these metrics is considered best practice during a technical SEO audit.


What is Largest Contentful Paint?

Largest Contentful Paint (LCP) is a performance metric that measures the time it takes to render the largest content element visible in the viewport. It could be an image, video, or any sizable text block.

LCP is a key indicator of perceived load speed because it illustrates the point when the main content of a page has likely loaded – a moment critical to user experience.

For a good user experience, Google recommends an LCP of 2.5 seconds or less.


Optimizing your Largest Contentful Paint

Improving LCP involves efforts at various levels, revolving primarily around improving server response times, optimizing the critical rendering path, and managing resource load prioritization.

Understanding these aspects will allow you to devise strategies that effectively address the root causes of a high LCP.

It’s worth noting that even a slight improvement in LCP can lead to a significant reduction in bounce rates. Its techniques also majorly overlap with practices used for website speed optimization.

# Eliminate resource load delay

One effective strategy to improve LCP is eliminating potential delays in loading essential resources. These could be large images, videos, or CSS files that block rendering.

Techniques such as lazy loading, which loads images only when needed (as a user scrolls down), can be beneficial. Compressing large files and using Content Delivery Networks (CDNs) to reduce load times is also helpful.


# Eliminate element render delay

Sometimes, render-blocking elements can hinder LCP.

By deferring non-critical CSS and JavaScript, you can speed up the rendering of the main content. Inlining critical CSS and deferring the rest can also help improve LCP.


# Reduce resource load time

Reducing resource load time is another key strategy for optimizing LCP.

Minify and compress text files and images and preconnect to important third-party requests to reduce the overall load time.

Improving load times is also crucial to crawl budget optimization.


# Reduce Time to First Byte

Reducing Time to First Byte (TTFB) – the time it takes for a user’s browser to receive the first byte of page content – can significantly impact LCP.

To do this, you can optimize server response times, implement server caching, and use a CDN to serve your content from a location nearest to your user.

Digital excellence demands a keen focus on the user experience, and Core Web Vitals are an integral part of that narrative. That’s why we encourage all our clients to see these metrics not as mere indicators but rather as the pulse of their online presence. Mike Jackowski COO, ASPER BROTHERS Let's Talk


What is First Input Delay?

First Input Delay (FID) measures the time from when a user first interacts with your page (like clicking a link or tapping a button) to the time the browser can respond to that interaction.

FID is a crucial metric for understanding the interactivity and responsiveness of your website.

Google suggests a target FID of less than 100 milliseconds for a quality user experience.


Optimizing your First Input Delay

Optimizing FID involves reducing the impact of third-party code, minimizing JavaScript execution, and prioritizing input-ready states over page loading.

Remember, a website that responds quickly to user inputs is a website that retains user engagement.

According to an Akamai study, a 100-millisecond delay in website load time can hurt conversion rates by 7%.

# Manage first-party script execution

Heavy JavaScript execution can block the main thread, which in turn can delay the browser’s response to user input. To improve your site’s interactivity, minimize the execution time of your JavaScript.

You can achieve this by shipping only the code that users need. Breaking up long tasks into smaller, asynchronous ones can also help keep your main thread free and responsive to user inputs.


# Improve interaction readiness

Improving interaction readiness can include a variety of tactics, such as designing for mobile-friendliness, streamlining your page’s resources, and ensuring the usability of visual elements.

Optimizing a page for interaction readiness can lead to improvements in FID as users will find the page more responsive.


# Improve interaction readiness with data-fetching

Effective data-fetching strategies like prefetching, preloading, or even prerendering can significantly improve FID.

The goal here is to have the data ready before the user asks for it, making the interaction appear instant when a user finally triggers an action.


# Reduce interaction latency with third-party script execution

Delaying the execution of non-critical third-party scripts until after the main content is interactive can also improve FID.

Reducing latency includes delaying the execution of scripts for analytics, ads, and widgets that aren’t critical during the initial page load.


# Use a Web Worker

Web Workers allow you to run JavaScript on a background thread, leaving the main thread free to remain responsive to user input.

While adding Web Workers can be complex depending on the JavaScript being offloaded, it can significantly improve FID when done correctly.


# Use browser caching

Browser caching can help load assets more quickly, leading to improvements in FID.

By storing a version of your site in the user’s browser, you can ensure that not all assets need to be loaded from scratch upon each visit.


# Implement lazy loading

Lazy loading can prevent offscreen or hidden content from slowing down the page load speed, allowing users to interact with your site more quickly.


What is Cumulative Layout Shift?

Cumulative Layout Shift (CLS) measures the instability of content on a page.

In simple terms, CLS quantifies how much elements move around on the screen during page load.

A high CLS is detrimental to the user experience, as it can cause users to click unintended buttons or lose their place while reading.

Google recommends a CLS score of 0.1 or lower for a good user experience.


Optimizing your Cumulative Layout Shift

Optimizing CLS involves ensuring elements on your page don’t shift unexpectedly.

This means taking care of images and ads, dynamic content, web fonts, and actions waiting for a network response.

Creating a well-ranking website requires a thorough understanding of these elements and implementing strategies to minimize layout shifts.

# Set the default aspect ratio of images based on width and height attributes

Setting width and height attributes on images and video elements can prevent layout shifts.

When both attributes are specified, the browser can allocate the correct amount of space on the page while the image is loading, preventing unexpected jumps.


# Incorporate dynamic ad sizes

Reserving space for ads is crucial. You can do this by statically committing the largest possible area for the ad slot or by using dynamic ad sizes.

When the browser knows the space needed for the ad in advance, it won’t cause the layout to shift when it finally loads.


# Stay clear of embeddable widgets

Embeddable widgets like iframes can cause layout shifts if their size isn’t specified in advance.

You can prevent unexpected layout shifts by providing sufficient space for these elements before they load.


# Avoid inserting new content above existing content

Avoiding unexpected content shifts also involves not inserting new content above existing content unless in response to user interaction.

This ensures that the view isn’t pulled away from the user unexpectedly.


# Reduce font-related CLS

Web fonts can cause a significant layout shift, especially when they replace a fallback font once loaded.

To minimize such shifts, you can use techniques like font display swap.


# Avoid animations of properties that trigger layout changes

Avoiding animations that trigger layout changes can also help optimize CLS.

Choose transform animations over animations of properties like width, height, or position, which may cause layout shifts.


# Ensure your site is eligible for the back/forward cache

Eligibility for the browser’s back/forward cache (bfcache) can also help reduce layout shifts.

When pages are stored in the bfcache, they can load instantly and without layout shifts when a user navigates to them.


How to check your Core Web Vitals?

There are various tools that can help you perform a comprehensive Core Web Vitals audit.

They can provide a holistic view of your website’s performance and user experience, helping you identify areas for improvement.

# PageSpeed Insights

PageSpeed Insights

PageSpeed Insights is a Google tool that provides both lab and field data about a page’s performance.

It offers a comprehensive performance score, including Core Web Vitals, and provides helpful suggestions on improving performance.


# ScreamingFrog


ScreamingFrog is an SEO tool that allows for in-depth website audits, including finding potential issues related to Core Web Vitals.

Its integration with PageSpeed Insights allows for comprehensive performance audits.


# Google Search Console

Google Search Console

Google Search Console provides a Core Web Vitals report with grouped issues affecting your site’s performance.

This tool can help you understand how your site performs during real-world usage and guide your optimization efforts.


# Chrome DevTools

Chrome DevTools

Chrome DevTools offers a Performance panel to check LCP, CLS, and other performance metrics in a lab setting.

It provides detailed breakdowns of what’s happening as your page loads and renders, helping you pinpoint potential bottlenecks.


# Google Lighthouse

Google Lighthouse

Google Lighthouse, available as a Chrome extension or within Chrome DevTools, is an automated tool for auditing performance, accessibility, and more.

Lighthouse’s performance audit includes Core Web Vitals, making it a valuable tool for detailed website audits.



Core Web Vitals have become a crucial aspect of delivering superior user experience and enhancing the visibility of your website in search engine results.

A thorough understanding of these metrics – LCP, FID, and CLS – and effective strategies to optimize them will equip you with the means to drive meaningful improvements.

By leveraging various tools, you can perform a comprehensive Core Web Vitals audit and ensure your website provides the smooth, fast, and engaging experience that users expect and demand.



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