Optimize the LCP Resource Load Duration

Optimize the LCP Resource Load Duration

Largest Contentful Paint (LCP) is a crucial web performance metric that measures how quickly the largest contentful element becomes visible within the viewport. The resource load Duration is a sub-part of the LCP that indicates how much time is lost fetching net network resource for the LCP element. Let's dive deep into the resource load duration aspect of LCP and explore its importance and optimization strategies.

What is Resource Load Duration in LCP?

Resource Load Duration, also known as Load Duration, is the time it takes for the browser to download thenetwork resource (eg the image) that will later become the LCP element. 

For image-based LCP elements, the load duration represents the time between when the LCP element starts loading and when the browser finishes downloading the image. If the LCP element is not an image (e.g., a text block), the load duration is typically 0 milliseconds.

Resource Load Duration is measured from the moment the browser starts downloading the LCP resource until it has finished downloading. This measurement is crucial because it directly impacts how quickly users can see and interact with the main content of a webpage. The duration can be influenced by several factors, including:

• File Size: Larger files take longer to download.
  
• Network Speed: Slower network connections increase download times.
  
• Server Response: The efficiency of the server in delivering resources affects load times.
  
• Concurrent Downloads: Other resources being downloaded simultaneously can compete for bandwidth, affecting load duration
  

How to detect resource load Duration

There are 2 easy ways to detect resource load delay.

Network Inspection in Chrome DevTools: Use the Ctrl + Shift + I shortcut to open Chrome's Developer Tools, then select the "Network" tab and reload the page. Look up the LCP element. The network inspector will show you how long it took to download the resrource. 

pro tip: if you enable large request rows in chrome devtools you can also see the LCP latency, transferred size and actual size!

Leverage Real User Monitoring (RUM) Data: Real User Monitoring tools often log LCP attribution data. With RUM, you can visualize the breakdown of LCP sub-parts (over time or by page), giving you a clear picture of the load delay for LCP elements across your entire site or per page. The example below shows a global LCP breakdown along with the corresponding load duration.

How to LCP Improve Load Duration

A resource load delay happens when the download order and timing of resources aren't optimal. There are, in essence,  two straightforward ways to fix this: send less data or send data more efficiently. Lets explore some common patterns:

Optimize file size:

When it comes to optimizing image file sizes without compromising on quality, you’ve got three go-to moves:

Use Modern Image Formats: AVIF and WebP are the reigning champions here. AVIF is the heavy-hitter in compression, shrinking file sizes up to 50% smaller than WebP for equivalent quality in complex photos, all while handling color depth and detail like a pro. For simpler images, like icons or illustrations, WebP often has the upper hand in terms of browser compatibility and faster decoding, but AVIF tends to deliver more bang for the byte overall, especially for photos and more complex visuals.

Implement Responsive Images: The <picture> element and srcset attribute are your secret weapons for serving optimized images based on screen size. This lets you deliver a different, tailored image based on a user’s viewport, meaning high-res versions go to large screens, and lighter versions load on smaller ones—keeping load times lean and snappy. Here’s a quick setup:

<picture>
  <source media="(min-width: 800px)" srcset="large.jpg 1x, larger.jpg 2x">
  <img src="photo.jpg" loading="lazy" alt="Description">
</picture>

Check Your File Dimensions: Just because your images are "responsive" doesn’t mean they’re right-sized. Make sure the dimensions of your images match how they’re displayed on the site: serving a 2000px wide image where a 500px one would do is a quick way to waste bandwidth.

In a nutshell, moving to formats like AVIF, implementing responsive image techniques, and double-checking your file dimensions will keep your images optimized and your load times fast. This combo can make the difference between a slick, high-speed experience and a site that drags its feet in loading. For AVIF vs. WebP, go AVIF if you want top-notch compression, but keep a fallback for WebP if your audience includes older browsers

Improve network performance: 

To tighten up network performance, a few advanced tactics can go a long way:

Caching: Leverage caching smartly, so repeat visitors aren’t wasting time re-downloading static resources. Once cached, those assets load instantly from local storage, bringing load times down to zero. Look into advanced configurations, like versioned URLs for updates, so assets don’t expire unnecessarily early. This setup doesn’t just boost load speed but also reduces server strain and bandwidth costs.

HTTP/3: This newer protocol is all about speed, using multiplexed connections and UDP to reduce latency. Unlike HTTP/2, which still relied on TCP, HTTP/3 minimizes delays and packet loss, making it especially effective in congested or spotty network environments.

Self-Hosting Resources: When you self-host key assets—think fonts, libraries, or analytics scripts—you sidestep the need to connect to third-party servers. This reuse of a single, already open connection reduces the overhead of establishing separate connections, speeding up load times by keeping everything in-house. Self-hosted resources also give you full control over compression and cache policies, eliminating risks from third-party outages and aligning all assets to your exact setup.

Optimizing these network settings is crucial for performance and a smoother user experience. Whether it’s adjusting caching, upgrading protocols, or centralizing resources, each step shaves off precious milliseconds.

Resource Prioritization

To nail down Largest Contentful Paint (LCP), cutting down on network competition is key:

Prioritize Critical Resources: Flag essential resources, like hero images or above-the-fold CSS, with fetchpriority="high". This signals the browser to download these assets first, keeping them from getting bogged down by scripts, widgets, or third-party elements that don’t need instant loading. Prioritizing these critical resources reduces load time for the content your users care about most.

Reduce Network Contention: Streamline initial downloads by deferring or lazy-loading non-essential assets. Postpone loading for any images or videos not immediately visible, as well as background or secondary elements. Using loading="lazy" for offscreen media is a good place to start, while further deferring other non-essential scripts and assets will free up bandwidth and cut down on any competition with your critical resources, keeping your page’s main content quick to load and display.

By focusing on what loads first and ditching the distractions, you give your LCP metrics the VIP treatment they deserve.