Understanding LCP: The Complete Guide to Largest Contentful Paint

Understanding LCP: The Complete Guide to Largest Contentful Paint

Master Largest Contentful Paint (LCP), the Core Web Vital that measures loading performance. Learn its history, how it works, why it matters, and proven strategies to optimize your LCP score.

Understanding LCP: The Complete Guide to Largest Contentful Paint

What is Largest Contentful Paint (LCP)?

Largest Contentful Paint (LCP) is a Core Web Vital metric that measures how long it takes for the largest content element visible in the viewport to fully render. In simpler terms, LCP tells you when the main content of your page has finished loading from a user’s perspective.

When someone visits your website, they’re watching for that moment when “the page is ready.” LCP captures that moment by tracking when the biggest piece of content (typically a hero image, featured video thumbnail, or large text block) becomes fully visible.

What Elements Are Considered for LCP?

The browser considers these element types when determining the LCP element:

  • Image elements (<img>)
  • Image elements inside SVG (<image> inside <svg>)
  • Video poster images (<video> with poster attribute)
  • Elements with background images loaded via url() in CSS
  • Block-level text elements containing text nodes (<h1>, <p>, <div>, etc.)

The browser continuously monitors the page and updates the LCP candidate as larger elements finish rendering, until the user first interacts with the page (click, tap, or keypress).

LCP Thresholds

Google defines three performance categories for LCP:

RatingTime
Good≤ 2.5 seconds
Needs Improvement2.5 - 4.0 seconds
Poor> 4.0 seconds

To provide a good user experience, Google recommends that sites aim for an LCP of 2.5 seconds or less for at least 75% of page visits.

The History of Largest Contentful Paint

The Problem with Previous Metrics

Before LCP, the web performance community used several other loading metrics, each with limitations:

Load Event (Oldest) The traditional load event fired when all resources finished loading. This was often irrelevant to user experience; a page could feel “ready” long before all background scripts and images loaded.

DOMContentLoaded This fires when the HTML is parsed, but before stylesheets and images load. Too early to represent when users see meaningful content.

First Paint (FP) and First Contentful Paint (FCP) Introduced by Google around 2017, FCP measures when the first content appears. However, this could be something trivial like a loading spinner or navigation bar, not the main content users came to see.

First Meaningful Paint (FMP) Google attempted to identify “meaningful” content automatically, but FMP was unreliable. It used complex heuristics that varied across page types and often produced inconsistent results.

Speed Index Speed Index measures how quickly content is visually populated. While useful, it’s computationally expensive and difficult to explain to non-technical stakeholders.

The Birth of LCP (2020)

In May 2020, Google introduced Core Web Vitals, with LCP as the loading performance metric. The Chrome team chose LCP because:

  1. It’s user-centric - Measures what users actually care about: seeing the main content
  2. It’s simple to understand - “How long until the biggest thing loads?”
  3. It’s reliably measurable - Can be measured in both lab and field conditions
  4. It correlates with user perception - Research showed LCP closely matched when users felt pages were “loaded”

Evolution of LCP (2020-2025)

2020-2021: Initial Rollout LCP launched as part of Core Web Vitals. Early tools and documentation helped developers understand the new metric.

June 2021: Ranking Factor LCP became an official Google ranking signal as part of the Page Experience update. Sites with good LCP scores gained competitive advantages.

2022: Refinements Google refined which elements counted toward LCP. For example, images with opacity: 0 or visibility: hidden were excluded, as were images that hadn’t finished decoding.

2023: Cross-Document Navigation Updates improved LCP measurement for single-page applications (SPAs) and sites using the Navigation API.

2024-2025: Prerender Support LCP measurement evolved to better handle prerendering and speculation rules, measuring from when the prerendered page becomes visible rather than when prerendering started.

Why LCP Matters

User Psychology and Perceived Performance

Users form impressions about your site within milliseconds. Research from Google and others shows:

  • 53% of mobile visitors abandon sites that take longer than 3 seconds to load
  • Users perceive pages with fast LCP as more trustworthy and professional
  • Slow LCP creates anxiety; users wonder if the site is broken

LCP directly measures that critical moment when users see your main content and decide whether to stay or leave.

Business Impact

Real-world studies demonstrate LCP’s impact on business metrics:

Vodafone: A 31% improvement in LCP led to an 8% increase in sales.

NDTV: Reducing LCP from 8 seconds to 3 seconds resulted in a 50% reduction in bounce rate.

Tokopedia: A 55% improvement in LCP correlated with a 23% increase in average session duration.

SEO Benefits

LCP is one of three Core Web Vitals that Google uses as ranking signals. While content quality remains paramount, LCP can be a tiebreaker between otherwise equal pages. In competitive niches, a good LCP score provides an edge.

How LCP is Measured

The Measurement Process

  1. Browser starts tracking when the page begins loading
  2. Candidates are identified as content elements render
  3. LCP updates whenever a larger element becomes visible
  4. Measurement stops when the user first interacts (click, tap, scroll, keypress)
  5. Final LCP is the render time of the largest element seen

Lab vs. Field Measurement

Lab Data (Synthetic Testing)

  • Collected in controlled environments
  • Consistent, repeatable results
  • Useful for debugging and development
  • Tools: Lighthouse, WebPageTest, Chrome DevTools

Field Data (Real User Monitoring)

  • Collected from actual user visits
  • Reflects real-world conditions (devices, networks, locations)
  • What Google uses for rankings
  • Tools: Chrome UX Report (CrUX), PageSpeed Insights (field data), Google Search Console

Important Caveats

User Interaction Stops Measurement If a user clicks something before the main content loads, LCP measurement stops at that point. This can mask slow loading issues.

Single-Page Applications SPAs that don’t trigger full page loads may need additional instrumentation to measure LCP accurately for route changes.

Iframes Content inside iframes isn’t considered for the parent page’s LCP.

Common Causes of Poor LCP

1. Slow Server Response Time (TTFB)

If your server takes too long to respond, everything else is delayed. Time to First Byte (TTFB) directly impacts LCP.

Symptoms:

  • High TTFB in Lighthouse or WebPageTest
  • Slow initial HTML delivery

Solutions:

2. Render-Blocking Resources

CSS and synchronous JavaScript in the <head> block rendering until they’re downloaded and processed.

Symptoms:

Solutions:

3. Slow Resource Load Times

The LCP element itself (usually an image) takes too long to download.

Symptoms:

  • Large LCP element file size
  • Images loading from slow third-party servers
  • No image optimization

Solutions:

  • Compress images (use tools like Squoosh or ImageOptim)
  • Use modern formats (WebP, AVIF)
  • Implement responsive images with srcset
  • Serve images from a CDN
  • Preload the LCP image

4. Client-Side Rendering Delays

JavaScript frameworks that render content on the client can delay LCP significantly.

Symptoms:

Solutions:

Optimizing LCP: Proven Strategies

Optimize Your LCP Element

For Images:

<!-- Preload the LCP image -->
<link rel="preload" as="image" href="/hero-image.webp">

<!-- Use fetchpriority for critical images -->
<img src="/hero-image.webp" fetchpriority="high" alt="Hero image">

<!-- Avoid lazy loading the LCP image -->
<img src="/hero.webp" loading="eager"> <!-- NOT loading="lazy" -->

For Text:

<!-- Preload critical fonts -->
<link rel="preload" as="font" type="font/woff2"
      href="/fonts/main.woff2" crossorigin>

<!-- Use font-display: optional or swap -->
<style>
@font-face {
  font-family: 'Main Font';
  src: url('/fonts/main.woff2') format('woff2');
  font-display: swap;
}
</style>

Reduce Server Response Time

# Enable gzip compression (nginx)
gzip on;
gzip_types text/plain text/css application/json application/javascript;

# Enable caching headers
location ~* \.(jpg|jpeg|png|gif|ico|webp|avif)$ {
    expires 1y;
    add_header Cache-Control "public, immutable";
}

Eliminate Render-Blocking Resources

<!-- Defer non-critical CSS -->
<link rel="preload" href="non-critical.css" as="style"
      onload="this.onload=null;this.rel='stylesheet'">

<!-- Async scripts -->
<script src="analytics.js" async></script>

<!-- Deferred scripts -->
<script src="app.js" defer></script>

Use a CDN

Content Delivery Networks serve assets from servers geographically close to users, dramatically reducing latency. Popular options:

Implement Resource Hints

<!-- Preconnect to critical third-party origins -->
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://cdn.example.com">

<!-- DNS prefetch for less critical origins -->
<link rel="dns-prefetch" href="https://analytics.example.com">

<!-- Preload critical resources -->
<link rel="preload" href="/critical.css" as="style">
<link rel="preload" href="/hero.webp" as="image">

Measuring and Monitoring LCP

Tools for LCP Analysis

PageSpeed Insights

  • Enter any URL for instant analysis
  • Shows both lab and field data
  • Identifies the LCP element
  • Provides optimization suggestions

Chrome DevTools

  1. Open DevTools (F12)
  2. Go to Performance tab
  3. Record a page load
  4. Look for the LCP marker in the timeline

Lighthouse

  1. Open DevTools
  2. Go to Lighthouse tab
  3. Run an audit
  4. Review LCP score and diagnostics

Google Search Console

  1. Navigate to Core Web Vitals report
  2. See field data aggregated across your site
  3. Identify pages needing improvement

Setting Up Real User Monitoring

// Using the web-vitals library
import {onLCP} from 'web-vitals';

onLCP(metric => {
  console.log('LCP:', metric.value);
  // Send to your analytics
  sendToAnalytics({
    name: metric.name,
    value: metric.value,
    id: metric.id,
    entries: metric.entries,
  });
});

LCP Debugging Checklist

When troubleshooting poor LCP, work through this checklist:

  • Identify the LCP element using DevTools or PageSpeed Insights
  • Check server response time (TTFB should be under 600ms)
  • Look for render-blocking resources in the <head>
  • Verify the LCP image is optimized (compressed, proper format)
  • Ensure LCP image isn’t lazy-loaded
  • Check if LCP image is preloaded
  • Verify CDN is configured correctly
  • Review JavaScript execution time
  • Check for layout shifts that might delay LCP
  • Test on real mobile devices, not just throttled desktop

Common LCP Mistakes to Avoid

1. Lazy Loading the LCP Image

Never use loading="lazy" on above-the-fold images, especially your LCP element.

2. Using CSS Background Images for LCP

Background images are harder for the browser to prioritize. Use <img> tags for LCP elements when possible.

3. Ignoring Mobile Performance

LCP is measured separately for mobile and desktop. Many sites optimize for desktop but neglect mobile, where network conditions are often worse.

4. Overlooking Third-Party Impact

Third-party scripts can block rendering or compete for bandwidth. Audit and defer non-critical third-party resources.

5. Not Testing Real-World Conditions

Lab tests on fast connections don’t reflect user experience. Use field data and test on throttled connections.

Conclusion

Largest Contentful Paint is more than a metric. It’s a measure of how well your site delivers on user expectations. When someone visits your page, they’re waiting for that moment when the main content appears. LCP captures that moment and helps you optimize for it.

By understanding what LCP measures, why it was created, and how to improve it, you can deliver faster, more satisfying experiences that keep users engaged and help your site rank better in search results.

Remember: the goal isn’t just a good LCP score. The goal is happy users who find what they’re looking for quickly and easily.

FAQs

1. What’s a good LCP score? 2.5 seconds or less is considered “good.” Between 2.5-4 seconds needs improvement, and over 4 seconds is poor.

2. How do I find my LCP element? Use PageSpeed Insights or Chrome DevTools Performance panel. Both identify the specific element that determined your LCP.

3. Why is my LCP different in lab and field data? Lab data uses controlled conditions, while field data reflects real users with varying devices, networks, and locations. Field data is typically more variable.

4. Can LCP change during the page load? Yes. The browser updates the LCP candidate as larger elements render, until the user interacts with the page.

5. Does LCP affect Google rankings? Yes, LCP is part of Core Web Vitals, which is a ranking signal. However, it’s one of many factors, and content relevance remains most important.

6. Should I preload all my images? No. Only preload the LCP image and other critical above-the-fold resources. Preloading too much can be counterproductive.
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