Largest Contentful Paint (LCP)
Definition
Largest Contentful Paint (LCP) measures how long the largest image or block of text in the visible part of a page takes to render. Google's guidance for a good experience is within 2.5 seconds of the page starting to load, judged at the 75th percentile of visits.
On most pages the LCP element is the hero image, a large heading or a banner. What slows it is usually one of four things: a slow server response, scripts and stylesheets that block rendering, an oversized image, or a hero image set to lazy-load, which tells the browser to wait before fetching the very thing the visitor came to see. Resize and compress the hero image, and load it first.
Google assesses it at the 75th percentile of page loads, mobile and desktop separately, so a quarter of visits can be slower and the page still passes. Field data from real visits, shown in the Core Web Vitals report in Search Console, is what counts. A lab test from a fast office connection can look healthy while visitors on mobile data wait. Check the page on a phone away from the office network.
Image-led sites such as property, hotel and retail pages are exposed, because a large photo sits at the top of the page. Serve a smaller file to phones, and don't put a slider ahead of the content you want measured.
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