Largest Contentful Paint (LCP)
Largest Contentful Paint (LCP) is a user-centric metric for measuring perceived load speed. It marks the point in the page load timeline when the page's main content has likely loaded and becomes visible to the user. LCP is one of the Core Web Vitals introduced by Google to evaluate the user experience of a webpage. It's included in Google PageSpeed.
What?
Specifically, here's what LCP measures:
- Contentful: The metric focuses on content elements, such as images, videos, or block-level text elements (
< p >, < div >, etc.). - Large: It focuses on the visibility and size (viewport coverage) of the element. Essentially, it's looking at the largest visible element within the viewport.
- Paint: It refers to the point when the content is rendered (or painted) on the screen.
Why?
Why is LCP Important?
LCP provides insights into how quickly users are actually able to see and interact with the main content of a page. If the LCP is delayed, users might perceive the page as slow, even if other metrics (like First Contentful Paint) are fast. A slow LCP can be a result of various issues, such as slow server response times, slow resource load times, or client-side rendering issues.
Additionally, slow sites might get penalized by search engines.
Example: Loading spinner?
Maybe an example: Some years ago I worked on a site that had a loading spinner. This thingy was really annoying and maybe that was because it delayed the LCP: You could only interacte with the site when it was fully loaded and the spinner was gone.
Once this spinner was removed, the site felt much faster.
How to optimize LCP?
Some suggestions:
- Optimize Images: Ensure images are compressed and are in modern formats like WebP. Also, consider using responsive images to deliver the right size based on the user's device.
- Preload Important Resources: Use the preload directive to load key resources as soon as possible.
- 'Optimize Server Response Times: Consider a more powerful or better configured server or better caching. Using a Content Delivery Network (CDN) is usually mentioned here as an option, but I doubt if they work at all
- Avoid or Optimize Blocking Resources: CSS and JavaScript can block the rendering of a page. Ensure you're asynchronously loading non-critical CSS and JavaScript and inlining critical CSS when necessary
- Optimize Client-side Rendering: If you're using JavaScript frameworks, ensure that the rendering is efficient. This might involve techniques like server-side rendering or using static site generation for some pages.
- Optimize Web Fonts: If you're using custom fonts, consider font-display properties and preloading fonts.