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The Core Web Vitals Butterfly Effect: Small Optimizations, Big Impact On User Experience And Conversions

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In the fast-paced and highly competitive world of eCommerce, delivering an exceptional user experience is no longer a luxury; it's a necessity. With the introduction of Core Web Vitals, Google has provided a set of standardized metrics that quantify the user experience of a website.

For eCommerce businesses, understanding and optimizing these metrics is crucial to engage users, drive conversions, and stay ahead of the competition

Core Web Vitals focus on three key aspects: loading speed, interactivity, and visual stability. This article covers three key metrics—Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS)—and explores the impact of each of these metrics on eCommerce conversion rates as well as the trade-offs.

Core Web Vitals demystified

Largest Contentful Paint (LCP):

LCP measures the loading speed of a website by tracking the time it takes for the largest content element to become visible. In the eCommerce context, this could be a hero image showcasing a flagship product or a prominent call-to-action button.

A fast LCP is essential to capture user attention and prevent high bounce rates. Studies have shown that a mere one-second delay in page load time can lead to a staggering 7% reduction in conversions (BigCommerce).

Imagine an online fashion store that takes too long to display product images—potential customers are likely to abandon the site in favor of faster competitors.

Interaction to Next Paint (INP):

INP, which replaces First Input Delay (FID), measures the responsiveness of a website by capturing the latency between a user's interaction and the subsequent visual response.

In an eCommerce setting, this could be the time it takes for a product to be added to the cart after a user clicks the "Add to Cart" button or the responsiveness of a search bar as users type their queries.

A low INP ensures a smooth and seamless user experience, fostering engagement and encouraging transactions. A responsive website instills confidence in users, making them more likely to complete their purchases.

Cumulative Layout Shift (CLS):

CLS quantifies the visual stability of a website by measuring unexpected layout shifts during the lifespan of a page. For eCommerce websites, a high CLS can be particularly detrimental as it can lead to accidental clicks on unintended elements, such as adding the wrong product to the cart or clicking on an irrelevant link.

Ensuring a stable layout is vital to maintain user trust and facilitate seamless interactions. A visually stable website allows users to browse and transact with confidence, without the frustration of constantly readjusting their focus.

Tradeoffs and interplay between Core Web Vitals

While optimizing Core Web Vitals is essential, it's equally important to strike a balance between performance and functionality. Each metric is interconnected, and improving one may have consequences on the others.

For instance, techniques used to improve LCP, such as lazy-loading images, can inadvertently increase CLS if not implemented properly. Similarly, optimizing INP by deferring JavaScript execution may delay the loading of critical interactive elements, impacting the user experience.

An online electronics store, for example, may prioritize showcasing high-resolution product images and videos to provide a comprehensive product experience. However, this approach may impact LCP if not optimized properly. On the other hand, a ticketing website may prioritize INP to ensure that users can quickly and seamlessly select seats and complete their purchases without any delays.

Similarly an eCommerce website may decide to lazy-load product images to improve LCP. However, this implementation delays the loading of the "Add to Cart" button, which is crucial for user interaction. As a result, while the LCP improves, the INP suffers, leading to a slower response time when users try to add products to their cart. This tradeoff negatively impacts the user experience and may lead to lower conversion rates.

Another example is say an eCommerce website implements lazy-loading for product images without proper placeholder dimensions. As a result, the LCP improves, but the website experiences layout shifts as images load, leading to a higher CLS. This tradeoff can disorient users, as elements may suddenly move around the page, making it difficult to interact with the content and potentially leading to accidental clicks or missed conversions.

One final example, say an eCommerce website optimizes its JavaScript execution to improve INP by breaking down long-running tasks into smaller, asynchronous chunks. However, this approach causes certain elements to render later, resulting in layout shifts and a higher CLS. Users may experience a visually unstable page as elements appear and shift unexpectedly, leading to a disorienting experience and potentially impacting conversion rates.

Optimization strategies and associated tradeoffs

Image optimization

By compressing images, using appropriate formats like WebP, and lazy-loading non-critical images, eCommerce websites can significantly improve LCP. However, careful implementation is necessary to avoid negatively impacting CLS.

For instance, an online furniture store can optimize product images to load quickly while ensuring that the page layout remains stable during the loading process.

JavaScript optimization

Deferring non-critical JavaScript and implementing lazy-loading can enhance LCP, but it's crucial to ensure that critical interactions are not delayed, as this may increase INP.

An eCommerce website selling software products can prioritize the loading of essential features like product demos and pricing information while deferring less critical elements.

Content prioritization

Lazy-loading non-critical content, such as images and videos below the fold, can improve LCP by focusing on rendering critical content first. However, proper space reservation is essential to prevent layout shifts and maintain a low CLS.

An online grocery store can prioritize the loading of product categories and search functionality while lazy-loading product recommendations and user reviews.

Font optimization

Using techniques like font-display:swap and preloading critical fonts can improve LCP and prevent layout shifts, thereby enhancing CLS.

An eCommerce website selling luxury products can ensure that its brand fonts load quickly and seamlessly, providing a visually appealing and stable experience.

Third-party script optimization

Lazy-loading non-critical third-party scripts and using async or defer attributes can reduce their blocking time and improve LCP. However, careful consideration should be given to scripts that impact critical interactions to avoid increasing INP.

An online travel agency can optimize the loading of third-party widgets like customer reviews and social media feeds while ensuring that the booking process remains smooth and responsive.

Animation optimization

Leveraging CSS animations instead of JavaScript animations and implementing efficient techniques like FLIP and CSS transforms can improve INP and minimize layout shifts, positively impacting CLS.

An eCommerce website selling clothing can use smooth and visually appealing animations to showcase product variations without compromising the responsiveness of the site.


Mastering Core Web Vitals is not just a technical exercise; it's a strategic imperative for eCommerce businesses striving to deliver exceptional user experiences and drive conversions. By understanding the impact of LCP, INP, and CLS on user engagement and sales, eCommerce leaders can make informed decisions, prioritize optimizations, and strike the right balance between performance and functionality.

Investing in Core Web Vitals optimization is an investment in your eCommerce success. By delivering fast, responsive, and visually stable experiences, you can foster user trust, increase engagement, and ultimately boost conversion rates. Real-world examples from various eCommerce sectors demonstrate the tangible benefits of optimizing Core Web Vitals.

PhotonIQ Performance Proxy (P3) speeds up websites and apps for faster, easier customer browsing and purchasing. To learn more about how you can automatically optimize your website performance, please view a demo, or chat with an expert.

Neglecting Core Web Vitals is like leaving money on the table – optimize them to boost your eCommerce success.

Photo by Unsplash+ in collaboration with Francesco Ungaro.

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