9 Core Web Vitals Mistakes Killing Your Website Performance

Here’s a number that should scare you: Rakuten 24 saw a 53% increase in revenue per visitor after fixing their Core Web Vitals. That’s not a tiny optimization. That’s a revenue transformation.

Yet only 54.6% of websites currently pass Core Web Vitals. That means nearly half of all websites are delivering a subpar experience to their visitors—and paying the price in lost conversions, lower rankings, and frustrated users.

Google’s Core Web Vitals aren’t just a technical checklist. They’re a direct measure of how users experience your website. And in 2026, with AI-powered search increasingly pulling fast, performant sites into featured snippets, your Core Web Vitals performance is make-or-break for your visibility.

Here’s the thing: most website owners don’t even know they’re failing. The mistakes are invisible until you know what to look for. Let’s fix that.

1. Your Largest Contentful Paint Takes Too Long

Largest Contentful Paint (LCP) measures how long it takes for the largest visible element on your page to load. Google wants this under 2.5 seconds. Most small business websites blow past this without realizing it.

The culprit? You’re loading massive hero images that aren’t optimized. A 2MB PNG background image on a desktop might take 2 seconds on fiber, but on a mobile connection? It might never finish loading before the user bounces.

What to do instead: Compress all images to WebP format. Use responsive images with srcset to serve smaller versions to mobile devices. Implement lazy loading for images below the fold. For the hero image, consider using a modern image format like AVIF or a CSS gradient as a lightweight fallback.

Swappie, an electronics refurbisher, cut their load time by 23% and increased mobile revenue by 42% after improving their Core Web Vitals. That’s what optimized LCP looks like in practice.

2. Images Aren’t Being Lazy Loaded

Lazy loading is one of the simplest performance wins available, yet it’s one of the most commonly skipped optimizations. When you don’t lazy load images, the browser tries to download every image on the page simultaneously—even the ones at the bottom that the user hasn’t scrolled to yet.

For a page with 20 images, that’s 20 concurrent downloads fighting for bandwidth. Your LCP suffers. Your Time to Interactive suffers. Everything suffers.

What to do instead: Add the loading="lazy" attribute to all images below the hero section. This single line of HTML tells the browser to only download images when they’re about to enter the viewport. Most modern frameworks and CMS platforms support this out of the box.

3. Your JavaScript Is Blocking Rendering

JavaScript is powerful, but when it loads before your content, it creates a bottleneck. The browser has to download, parse, and execute your scripts before it can render anything meaningful. This delay is measured in milliseconds on fast connections, but it adds up—and on slower connections, it can mean the difference between a sale and a bounce.

Third-party scripts are often the worst offenders. Chat widgets, analytics tools, social media embeds, and marketing automation scripts all add JavaScript overhead. A single page can easily have 10+ third-party scripts loading synchronously.

What to do instead: Defer non-critical JavaScript using the defer or async attributes. Move third-party scripts to load after the page becomes interactive. Use a performance budget to limit the total JavaScript on any given page. Consider whether each third-party script is worth its performance cost.

4. Cumulative Layout Shift Is Making Your Site Feel Broken

Cumulative Layout Shift (CLS) measures how much your page’s layout shifts unexpectedly. You know that feeling when you’re about to click a button and then an ad loads above it and now you’re clicking something else entirely? That’s CLS, and it’s infuriating.

Google considers a CLS score above 0.1 to be poor. Anything above 0.25 is in serious trouble. Yet this is one of the easiest Core Web Vitals metrics to fix—once you know what’s causing it.

What to do instead: Always include explicit width and height attributes on images and video elements. Reserve space for ads and embedded content before they load. Use the CSS aspect-ratio property to maintain consistent dimensions. Font loading can also cause CLS—use font-display: swap to prevent invisible text shifting when fonts load.

5. Your Server Response Time Is Too Slow

Before any of your HTML, CSS, or JavaScript can load, the server has to respond to the initial request. If your Time to First Byte (TTFB) is over 600 milliseconds, everything downstream is delayed. This is often the most overlooked Core Web Vitals issue because it’s invisible in the browser—it happens entirely on the server.

What to do instead: Choose a hosting provider with servers close to your target audience. Enable server-side caching to serve pre-built pages instead of generating them on each request. Consider a content delivery network (CDN) to cache your assets globally. Optimize your database queries if you’re using a dynamic CMS. For WordPress sites, caching plugins like WP Rocket or LiteSpeed Cache can dramatically improve TTFB.

6. You’re Not Using Modern Image Formats

If you’re still serving images as uncompressed JPEGs or massive PNGs, you’re throwing bandwidth away. Modern image formats like WebP and AVIF deliver significantly smaller file sizes with equivalent—or better—visual quality.

A 500KB JPEG can often become a 50KB WebP with no visible difference to the human eye. That’s a 90% reduction in download time for your largest assets.

What to do instead: Convert your image library to WebP or AVIF. Most image optimization tools (TinyPNG, Squoosh, ImageOptim) support these formats. If you can’t convert existing images, at minimum serve responsive versions with proper compression. Use the picture element to serve modern formats to browsers that support them while falling back to JPEG for older browsers.

7. Interaction to Next Paint Is Too High

Formerly called First Input Delay (FID), Interaction to Next Paint (INP) measures how quickly your page responds when users try to interact with it. Click a button, scroll the page, type in a form—any of these interactions should feel instantaneous.

A poor INP score usually means JavaScript is hogging the main thread. Long-running scripts, excessive event listeners, and inefficient framework hydration can all cause the browser to freeze when users try to do something.

What to do instead: Audit your JavaScript for long-running tasks. Break up heavy computations using requestIdleCallback or web workers. Minimize the use of heavy JavaScript frameworks on pages where they aren’t necessary. Remove unused JavaScript—your pages don’t need to load code for features that never execute.

RedBus saw a 7% increase in sales after improving their INP by 72%. That’s real revenue, from faster interactions.

8. You Have Too Many Third-Party Scripts

It’s tempting to add every tool that promises to help your marketing. Analytics, heatmaps, chat widgets, retargeting pixels, social proofs, email popups—the list goes on. But each script adds overhead, and they add up fast.

A page with 15 third-party scripts might load fine on a developer’s high-speed connection. But on a mobile device on a 4G network? Those scripts can add 5+ seconds of total blocking time.

What to do instead: Audit every third-party script on your site. Ask: does this directly contribute to conversions? If it’s only for analytics or “nice to have” features, consider removing it or loading it asynchronously after the page is fully loaded. Use a tag management system to control when scripts fire. For chat widgets, consider making them load only after the user scrolls or waits for a few seconds.

9. You Haven’t Tested on Mobile

All Core Web Vitals metrics are weighted toward mobile experience. Google’s indexing is mobile-first, meaning it primarily uses the mobile version of your site for ranking. Yet most website testing happens on desktop browsers with fast connections.

The reality is that most of your visitors are on mobile devices, often on cellular networks with inconsistent speeds. If your site works beautifully on your office WiFi but drags on mobile, you’re optimizing for the wrong audience.

What to do instead: Test your site using Google’s PageSpeed Insights (https://pagespeed.web.dev/) which simulates mobile performance. Test on real devices with throttled network speeds in your browser’s developer tools. Pay special attention to LCP and INP on mobile, as these are where most sites see the biggest gaps between desktop and mobile performance.

The Bottom Line

Core Web Vitals aren’t optional extras—they’re the foundation of how users experience your website. Renault saw a 13% rise in conversions with just a 1-second LCP improvement. That’s the kind of impact we’re talking about.

The good news? Every single mistake on this list is fixable. You don’t need a massive budget or a complete redesign. You need to know what to optimize.

Start with a free audit using Google’s PageSpeed Insights or Lighthouse. Pick the easiest fix on this list—the one that takes the least time—and implement it this week. Then move to the next.

Your visitors (and your revenue) will thank you.

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Ready to turn your slow website into a conversion machine? At YourWebTeam, we specialize in building websites that pass Core Web Vitals with flying colors while looking great and driving results. Get started today and let’s make your website work as hard as you do.