Largest Contentful Paint (LCP)

What is the Largest Contentful Paint (LCP)?

The Largest Contentful Paint is a measurement of the render time of the largest content element (of type image, video, or text) that has been painted on the visible part of the screen. The LCP timing indicates the time in milliseconds between requesting the page and when that largest contentful element is displayed on the visible part of the web page (above the fold).

History of the Largest Contentful Paint

When you think about it, the LCP might seem a trivial metric to represent the the loading part of the Core Web Vitals. Why not measure loading speed as 'the time it takes for the page to load'.

That is because it is hard (or even impossible) in most modern website to define when the page has loaded. More and more websites use techniques such as lazy loading or progressive loading where the page basically can keep loading forever. That means page speed cannot be measured by a single point in time.

There are several moments when loading the page that can cause a user to experience the page as fast or slow. For example the server delay (Time to First Byte), the first time that content is visible (First Contentful Paint), the time that the visible viewport may seem complete (Largest Contentful Paint) and when the page becomes interactive (when clicking on a link becomes possible) are all points during the loading process where the site may seem slow or fast!

The Largest Contentful Paint (LCP) is chosen because it focused on a visitor's user experience. When the LCP occurs you can assume that a visitor thinks the page is finished loading (even if behind-the-scenes processes are still running). The LCP was created to answer the question: 'When is the content of a page visible?'. This is why LCP is recognized as a core indicator of user-centric performance.

Why LCP Matters for Your Business

The Largest Contentful Paint is one of the 3 Core Web Vitals. As a Core Web Vital, the Largest Contentful Paint is important for SEO, but more importantly, LCP is critical for UX. Visitors do not like to be kept waiting, and with more and more mobile traffic (that tends to be slower than desktop traffic) optimizing the Largest Contentful Paint makes a lot of sense.

• For SEO. Yes, Google focuses on serving the best pages in its search results. LCP is part of Google's Core Web Vitals. Google clearly states that site speed is a factor in search results.
• For Visitors: According to recent research by Google itself, the probability of a user leaving the site doubles with a loading time of 3 seconds. You will probably recognize that for yourself. While surfing the internet almost nothing seems as annoying as a slow loading website. Chances are that you will quickly leave a slow loading page.
• Other reasons: Page Speed is a factor in your Google Ad Score. This means you can buy your ads cheaper and finally passing the Core Web Vitals is one one of the prerequisites for Google's top Story box.

A fast LCP gives the visitor the idea that the page loads quickly. As a result, a visitor is less likely to navigate away from the page.

Which Elements are considered LCP elements?

Not all elements are considered as an LCP element. The largest Contentful element has to be painted on the visible part of the screen (the viewport) before the user has interacted with the page.

The element has to be:

• An <img> element.
• An <image> element nested inside an <svg> element.
• A <video> element (the poster image or first video frame, whichever happens earlier, is used).
• An element with a background image loaded via the CSS url() function. (Note: This is an anti-pattern for LCP optimization as it makes the image undiscoverable to the browser's preload scanner).

• A block-level element containing text nodes or other inline text elements (in case of inline text elements like <span> the nearest block-level element <div> or <p> is considered).

Currently, certain elements are excluded as LCP candidates, such as elements hidden with opacity: 0, images that match 100% of the viewport size (cover images), and placeholders (low-entropy images). Keep in mind, this can change as the spec evolves!

Getting technical: measuring LCP Element Size

LCP identifies the largest visible content element in the viewport and calculates its size based on a set of precise rules. These rules ensure consistency and accuracy, even in complex layouts:.

• ViewPort only - Only the visible part of the page is considered. If an element is only partially in the visible viewport the considered size will be clipped.
• No Border, padding or margin. For all elements, text and image borders, padding and margins are completely ignored.
• Text size - text elements size get's reported as the smallest rectangle that can be painted around the text node(s).
• Image size - for images the smallest of the intrinsic dimensions (the original width and height) and the display size (the size on screen) is used to calculate LCP element size
• First size counts - when layout changes or when an element size changes only the first size is considered for the LCP
• Removed elements are included - when an element is removed from the DOM it is still an LCP candidate

Dynamic Nature of LCP

Largest Contentful Paint (LCP) is a dynamic metric. While rendering can be complex and occur in stages, it's normal for the LCP element to change during page load. Before the first user interaction, the browser's performance observer identifies all elements that could be considered LCP candidates. If a new element is rendered that is both visible within the viewport and larger than the previously identified LCP element, it becomes the new LCP..

Take-aways from LCP field data: At CoreDash we track millions of LCP entries per day. As it turns out, for mobile pageviews the LCP element is often a text-based element, either a paragraph or a heading. On average (or at the 75th percentile to be precise) the Core Web Vitals will pass when the LCP element is a text node or even a normal image. When the LCP element is a background image, video, or a lazy-loaded image, the Core Web Vitals tend to fail.

What is a good LCP score?

To pass the Core Web Vitals for the Largest Contentful Paint at least 75% of your visitors need to have a 'good' LCP score. A LCP score between 0 and 2.5 seconds is considered a good LCP score, a LCP score between 2.5 and 4 seconds needs improvement and a LCP score above 4 seconds is considered poor.

How LCP is Measured: The Four Key Phases

According to Google the Largest Contentful Paint can be broken down into 4 sub parts.

A page's final LCP time is not a single, monolithic value. It is the sum of four distinct sub-parts. Understanding this breakdown is the key to diagnosing and fixing LCP issues efficiently.

Here's a breakdown of the four phases:

• Time to first byte (TTFB) - This is pure server response time. It's the time it takes for the browser to receive the first byte of data from your server. A slow TTFB is a foundational problem that will always kill your LCP.
• Resource load delay - This is the "discovery gap." It's the time between TTFB and when the browser starts downloading the LCP resource. A long delay here means the browser found the LCP resource late, a classic symptom of using CSS background images or client-side rendering.
• Resource load time - This is how long it takes to actually download the LCP resource file (e.g., the image or font). This is all about file size and network conditions.
• Element render delay - This is the final delay. It's the time between when the LCP resource finishes downloading and when the element is fully rendered. This delay is almost always caused by the browser's main thread being blocked by other tasks, especially JavaScript processing.

Each of these focus areas can be optimized to improve the Largest Contentful Paint. To understand the steps you need to take, read Fix & Identify Largest Contentful Paint (LCP) issues.

How to measure the Largest Contentful Paint?

The Largest Contentful Paint (LCP) can be measured with pure JavaScript, Lab data, and Field tools. Both have their advantages and disadvantages.

Measure the Largest Contentful Paint with JavaScript

To measure Largest Contentful Paint (LCP) using JavaScript, the Performance Observer API offers a quick solution. The following code snippet demonstrates how to capture the LCP timing and element information:

new PerformanceObserver((list) => {
    const lcpEntry = list.getEntries().at(-1);
    console.log('LCP value: ', lcpEntry.startTime);
    console.log('LCP element: ', lcpEntry.element, lcpEntry.url);
  }).observe({ type: 'largest-contentful-paint', buffered: true });

This snippet tracks the LCP entry as it’s reported, displaying its timestamp and element details in the console. This snippet tracks the LCP entry as it’s reported. For more extensive insights, consider using the Web Vitals Library.

Measure Largest Contentful Paint (LCP) in Chrome DevTools

1. Open Chrome DevTools by pressing Ctrl+Shift+I (or Cmd+Option+I on Mac).
2. Navigate to the Performance tab.
3. Reload the page to view the Core Web Vitals.

The devtools performance tab now displays information about Core Web Vitals, including the timing and element of the Largest Contentful Paint.

Measure the Largest Contentful Paint with Lab and Field tools

Let's be clear: Lab and Field data serve two different purposes. You need both.

• Field data (RUM and CrUX) is the only data that actually matters for passing Core Web Vitals. It's what your real users experience. Google uses this data from its CrUX dataset. You start here to find out if you have a problem.
• Lab data (Lighthouse, etc.) is a controlled test. It's not what Google uses for ranking, but it's essential for debugging. You use this to figure out why you have a problem.

Here is a quick guide to the essential tools:

Improving the Largest Contentful Paint

Optimizing LCP requires a systematic approach that addresses the four phases. Anything that happens before the LCP element is painted, whether network-related or CPU-intensive, can impact the LCP score. Don't just chase one fix; understand the entire chain. Here is the high-level strategy:

• Optimize TTFB: Your server needs to be fast. If your TTFB is slow, nothing else matters. This involves server-side caching, using a CDN, and efficient backend code. Read more in our guide to optimizing TTFB.
• Eliminate Resource Load Delay: Ensure the LCP element is in the initial HTML so the browser's preload scanner can find it instantly. Avoid CSS background images for LCP. Preload critical images that are discovered late. Learn how in our guide to fixing Resource Load Delay.
• Reduce Resource Load Time: Make the LCP file smaller. This means using modern image formats like AVIF, responsive images, and proper compression. See our complete guide on how to Optimize the LCP Image.
• Shorten Element Render Delay: Stop blocking the main thread. Defer non-critical JavaScript, break up long tasks, and minimize render-blocking CSS. This is covered in our guide to fixing LCP issues.