Largest Contentful Paint

Largest Contentful Paint (LCP) is a Core Web Vitals metric that measures the time it takes for the largest visible content element (image, video, or text block) to render on the screen from when the page first starts loading.

Meaning & Context

LCP is part of Google's Core Web Vitals and focuses on perceived load speed from the user's perspective. A good LCP score is 2.5 seconds or less, while anything over 4 seconds is considered poor. The metric specifically tracks the largest element within the viewport, which typically represents when the main content has loaded.

Why It Matters

LCP directly impacts user experience and SEO rankings, making it critical for A/B testing page speed optimizations. Poor LCP scores can lead to higher bounce rates and lower conversion rates, as users perceive slow-loading pages negatively. When running experiments that modify page layouts or content, monitoring LCP ensures that performance improvements don't come at the cost of user experience.

Example

An e-commerce site testing two product page variants notices that Variant B, which uses larger hero images, has an LCP of 4.2 seconds compared to Variant A's 2.1 seconds, explaining why Variant B shows a 15% higher bounce rate despite having more visually appealing design.