Largest Contentful Paint explained: How to measure and improve LCP

The loading time of a website is crucial for user satisfaction and search engine ranking. Largest Contentful Paint (LCP) is an important metric that measures how long it takes for the largest visible content of a page to load. This article explains how LCP is measured and what steps can be taken to optimize this value.

What is Largest Contentful Paint (LCP)?

Largest Contentful Paint (LCP) is a metric introduced by Google as part of Core Web Vitals to evaluate the loading speed and user-friendliness of a website. LCP measures the time it takes from the start of page loading to the complete rendering of the largest visible content on the screen. This content can be an image, a video, or a large text block element.

The significance of LCP lies in its direct impact on user experience. When the largest visible content of a page loads quickly, users feel more satisfied and stay longer on the website. Additionally, search engines like Google prefer websites with good LCP values, which can lead to better rankings in search results.

Overall, Largest Contentful Paint (LCP) provides a realistic assessment of when the essential information on a page becomes accessible to the visitor. Each performance metric has its own significance and use cases, all of which collectively help to gain a comprehensive understanding of a website's loading time and user-friendliness.

What Factors Influence LCP?

Largest Contentful Paint (LCP) is influenced by various factors that affect both server-side and client-side aspects of website performance:

Server Response Time

A slow server response delays the moment when the first byte of the webpage is sent to the browser, thus extending the overall loading time. The choice of web hosting provider and the quality of the infrastructure (e.g. Dedicated Dedicated Servers vs. Shared Web Hosting) directly impact the server speed.

Render-Blocking Resources

JavaScript, CSS scripts, and stylesheets that are not loaded asynchronously or deferred block the rendering of the webpage until they are fully loaded and processed. Large and complex CSS or JavaScript files require more time to load and process.

Large Image and Video Files

Images that are not compressed or in an efficient format (e.g. WebP) slow down loading times. Loading images and videos immediately, instead of waiting to load them until they enter the user's visible area (Lazy Loading), extends the loading time.

Client-side Rendering

If extensive JavaScript operations need to be performed before content can be rendered, this delays the LCP. Large and complex DOM structures require more time to render.

Network Requests

The physical distance between the geographical location of the server and the user's device can increase latency times. Slow internet connections prolong content loading times.

Caching and CDN Usage

Without effective caching, resources need to be reloaded on each page visit, increasing loading times. Utilising a Content Delivery Network (CDN) can reduce loading times by distributing content through servers closer to the user geographically.

How to Measure LCP?

There are various methods and tools to accurately measure LCP:

Browser Developer Tools

Within the Chrome browser, LCP measurement can be done as follows:

• Open Developer Tools (right-click on the page > "Inspect" or press F12).
• Navigate to the "Lighthouse" tab and perform a performance audit.
• In the "Performance" tab, LCP can be directly measured and analysed by reloading the page and starting the recording.
• The LCP value is displayed in the waterfall diagram, along with a marker indicating when LCP occurs.

Online Tools

Using Google's own PageSpeed Insights online tool, LCP measurement can be done as follows:

• Open the tool at https://pagespeed.web.dev/
• After entering the URL, a report on Core Web Vitals will be generated, containing both field and lab data on LCP.

Also, some other online tools, such as the Uptrends Webspeed Test, display metrics for the LCP:

• Open the tool at https://www.uptrends.com/tools/website-speed-test
• After entering the URL, a report on the performance data is generated, which also includes data on Core Web Vitals, such as the LCP.

Difference between Field and Lab Data for LCP

Field Data are data collected from real users under real conditions, for example, via the Chrome User Experience Report (CrUX). These data are particularly valuable as they reflect the actual user experience. In contrast, Lab Data are collected under controlled conditions, for example, through tools like Lighthouse. These tests are reproducible and provide consistent metrics, but may not always be representative of all user experiences.

Discover which web hosting provider performs best in our performance test for LCP:

How to Improve LCP?

Implementing various measures can help optimise the LCP:

Optimising server performance

• Deployment of a Content Delivery Networks (CDN): A CDN distributes content across multiple servers worldwide, reducing loading times by allowing users to retrieve content from a server closer to their geographical location.
• Server-side optimizations: Enabling server-side caching to serve repeated requests faster. Optimising database queries to shorten response times. Enabling Gzip or Brotli compression to reduce the size of transmitted data.

Minimising Render-Blocking Resources

• Optimising CSS and JavaScript: Inline critical CSS: Only inline the necessary CSS for the initial page load to speed up rendering.
• Asynchronous and deferred loading of JavaScript: Load non-critical JavaScript files asynchronously or with the defer attribute to prevent blocking rendering.
• Avoiding large CSS and JavaScript files: Divide files into smaller, more modular parts and only load what is essential.

Optimising Images and Other Large Content

• Image compression: Compress images losslessly or with lossy compression to reduce file size. Scale images to the actual required dimensions to avoid oversized files.
• Modern image formats: Use modern image formats like WebP, which offer smaller file sizes with the same quality.
• Lazy Loading: Implement Lazy Loading for images and videos so they are only loaded when they enter the user's visible area.

Improving Client-Side Rendering

• Reducing DOM size and complexity: Simplify the DOM: Avoid unnecessary elements and nested structures to speed up rendering.
• Efficient use of web fonts: Load only the required font styles and sizes and use prefetching to reduce loading times.
• Prioritising visible content: Prioritise elements that should be visible first to ensure they are loaded quickly.

Caching and Browser Optimisations

• Browser caching: Set proper cache headers to keep resources in the browser cache and avoid repeated downloads.
• Service Worker: Use Service Workers to store page content in the cache and enable instant loading times on repeat visits.

Largest Contentful Paint (LCP) is a crucial metric for evaluating the loading speed and user experience of a website. By making targeted optimizations, such as improving server performance, minimising render-blocking resources, and efficient image compression, LCP can be significantly enhanced. Continuous monitoring and optimization of LCP values help to increase user satisfaction and achieve better rankings in search engines.

Find out about the fastest web hosting providers in Germany and how they perform in terms of Largest Contentful Paint (LCP).

Our article is based on our own experiences and research, as well as information from external sources.

Sources & further links on the topic:
https://www.uptrends.com/tools/website-speed-test (Uptrends Webspeed Test)
https://pagespeed.web.dev/ (Google Pagespeed Insights)
https://developers.google.com/search/docs/appearance/core-web-vitals?hl=de (Information on Core Web Vitals from Google)

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