![]() Wait, isn’t LCP just like the old onload metric? Not exactly. The page is not quite done, as some images have not yet loaded, but the user would probably start interacting with the page at this point, which is 2 seconds after start. This data came from various back-end services that returned asynchronously. The page is much more complete, showing that it knows about the user, their location, and the discounts Target wants to offer. As the page continues to load, the Largest Contentful Paint draws the screen out to this: Hopefully, this has occurred quickly enough that they aren’t distracted yet. This time is a proxy for when the user thinks the page is substantially complete, and that they can proceed with their task. The Largest Contentful Paint (LCP) is the time since navigation started when the largest amount of content is drawn on the screen, measured by pixel area. It doesn’t give the user anything to do yet, but they know that something is coming. In a typical case, about 300ms, which is noticeable, but feels very quick. ![]() The initial document request contains a basic skeleton of the page, which can be painted to the screen very quickly. Here is the First Contentful Paint today: It’s important to show the user something quickly to keep their attention and feel fast.įor example, let’s take a look at the site for a major US retailer,. This time represents the first visible indication to the user that their request has been received and that the content is coming. The First Contentful Paint (FCP) is the time between when the user starts navigating to a URL, via a link click or entering the address, and the time the first content is drawn on the screen. These are called the First Contentful Paint and the Largest Contentful Paint. In cases like this, there can be dozens or even hundreds of paint events.Įach paint event could be important for your application, but generally you care about two of them: the first time content was rendered, and the time the largest amount of content was rendered. But for many modern web applications, content will be broken into many smaller chunks and assembled asynchronously with JavaScript. For simple web pages, this happens only once: the HTML is downloaded, and the content is shown. Once a web browser has downloaded and parsed your content, it will render and paint the content to the screen. How to collect Core Web Vitals in your own application.The new emerging metrics for web performance, the Core Web Vitals, and what they measure.Some of the different ways modern websites can have poor performance.Finally, you’ll see how to interpret performance data to decide what your performance goals should be. You’ll also take a look at my Really Slow WebpageTM and implement a basic performance monitoring script to see just how bad it really is. ![]() This post will take a look at each of these metrics, what they measure, and how to use them. Google calls them the Core Web Vitals.Įach of the Core Web Vitals measures a different aspect of how a web application responds. Measuring this new slowness will take new metrics. You’d time how long a page takes to load, easy.īut the rise of client-side JavaScript has introduced bold new ways for websites to be frustratingly slow. ![]()
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