What Are Core Web Vitals?

Core Web Vitals are a set of metrics Google uses to measure real-world user experience. They became a ranking factor in 2021 and continue to influence search performance. The three core metrics are:

Largest Contentful Paint (LCP) measures how long it takes for the largest visible element to render. For many pages, this element is an image.

Interaction to Next Paint (INP) measures responsiveness — how quickly the page responds to user interactions.

Cumulative Layout Shift (CLS) measures visual stability — whether page elements shift unexpectedly as content loads.

Largest Contentful Paint (LCP)

Images are frequently the LCP element on content pages. A slow-loading hero image can push your LCP past the 2.5-second “good” threshold, hurting both user experience and rankings.

Optimize image loading priority by using the fetchpriority="high" attribute on your hero or above-the-fold image. This tells the browser to prioritize downloading this image over less critical resources.

Preload critical images in your HTML head:

<link rel="preload" as="image" href="hero.webp" type="image/webp">

Compress images aggressively while maintaining visual quality. Our compression tool helps you find the optimal balance.

Cumulative Layout Shift (CLS)

Images without explicit dimensions cause layout shifts. When a browser doesn’t know an image’s size in advance, it reserves zero space and reflows the page when the image loads, pushing content down.

Always include width and height attributes on images:

<img src="photo.webp" width="800" height="600" alt="Description">

Modern browsers use these attributes to calculate aspect ratios and reserve space before images load. For responsive images, CSS aspect-ratio properties achieve the same effect:

img { aspect-ratio: 4 / 3; }

Interaction to Next Paint (INP)

Large images can block the main thread during decode, delaying response to user interactions. The decoding="async" attribute lets the browser decode images off the main thread:

<img src="photo.webp" decoding="async" alt="Description">

Lazy loading below-the-fold images ensures they don’t compete for bandwidth with critical above-the-fold content. This reduces contention and improves responsiveness during the initial page load.

Measurement Tools

PageSpeed Insights provides Core Web Vitals scores for any URL along with specific recommendations for image optimization.

Chrome DevTools Performance panel records a timeline showing exactly when each image loads and how it affects LCP and CLS.

Google Search Console Core Web Vitals report shows real-user data from Chrome browsers, revealing how your actual visitors experience your site.

Monitor these metrics regularly and prioritize fixes that impact the most users. Image optimization is usually the highest-impact improvement for sites with visual content.