Which element is your Largest Contentful Paint?
A lab browser loads your page at desktop, phone, or tablet size and outlines the element that paints largest.
390 × 664
Example: Mona Lisa on Wikipedia, CC BY-SA 4.0
Watch every page's LCP with RumDash
Lab data vs. field data
What a lab check can and can't tell you
A lab check is good at simulating a first-time visit to a webpage. It loads your page in a real Chrome browser at a common screen size, waits for the layout to settle, captures the largest contentful paint element and metadata about the page load. In our case, the lab check outlines the element on a screenshot, and provides its CSS selector, HTML, and image metadata (when appropriate) so you can track the element down in your codebase.
But one load on a simulated device from a single location is not your audience. Real visitors use a variety of devices and screen sizes, and they're on spotty, unreliable networks. A fraction of these visitors are first-timers, while the rest are returning. Some visitors have account data and preferences that completely changes what they see. Those users' LCP element are often very different from the lab's.
That's why rumdash.io exists. To provide metrics from your real users.
Frequently asked questions
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It's the largest image, video, or block of text visible in the viewport when the page loads. The moment it finishes rendering is your Largest Contentful Paint (LCP) time, one of Google's three Core Web Vitals.
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Images, including an image inside an SVG, the poster or first frame of a video, an element with a CSS background image (gradients don't count), and block-level elements that contain text, such as headings and paragraphs. Chrome skips elements with an opacity of 0, elements that cover the whole viewport, and low-detail placeholder images.
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A responsive layout puts different things in view at each screen size. A hero image that dominates a desktop screen can shrink or drop below the fold on a phone, which leaves a headline as the largest element. Check each size your visitors use.
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A lab check is one load, on one device, from one location. Your visitors bring their own devices, networks, and cached files, and a scroll or tap ends their LCP early, so their LCP element and time can differ. For the real numbers, look up your site in the Core Web Vitals Checker, or monitor every page with RumDash.
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Start with the LCP element. If it's an image, make sure the browser can find it in the HTML, never lazy load it, add fetchpriority="high", and serve a compressed file sized for the screen. If it's text, cut render-blocking CSS and JavaScript and keep web fonts from hiding text while they load. A faster server response helps every LCP.
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Absolutely not! loading="lazy" tells the browser to wait until it knows the image is in view, which delays the fetch of the most important image on the page. Lazy load images below the fold instead.
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Many sites use bot protection that blocks lab browsers, and the result tells you when the site answered with a block page or an error status. A page also has to be public, so the lab can't open pages behind a login or on a private network.
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Results are kept for an hour, so everyone who checks the same page, screen size, and location in that hour shares one lab run. Check again after the hour is up.
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Yes. There's no sign-up, and you can check any public page.