![]() ![]() TTI can sometimes be misleading, but when combined with TBT, you get a clearer picture of how responsive your page is to user input.įor example, two pages can have the same TTI, but if one breaks work into smaller tasks while the other hogs the main thread with one very long task, the user experience will be worse in the second case. TBT quantifies how busy the main thread is before it becomes idle. TTI identifies when the main thread becomes idle. Total Blocking Time is measured only between First Contentful Paint and Time to Interactive. A page is considered to be “reliably interactive” if there are no long tasks on the main thread for at least five seconds. Time to Interactive (TTI) measures when the page becomes capable of reliably responding to user input. What is the difference between Total Blocking Time and Time to Interactive? Offload computationally intensive tasks to Service workers or Web workers.Where possible, break up code into functions that do less work and execute faster.Code-split your JavaScript bundles and lazy-load the ones that are not critical to the initial load.Tracking TBT is the first step towards improving page interactivity. Any time over that is counted towards that task’s blocking time. What is a long task?Ī task is long if it runs for more than 50 milliseconds on the main thread. TBT quantifies the time that the main thread spends tied up in long tasks, to estimate the risk of how much could long tasks potentially impact any user interaction that could happen during page load. If the main thread is blocked for longer than that, the user can experience delays and page unresponsiveness. If that delay is small, for example, less than 50 milliseconds, the user won’t even notice. If a user presses a button while the main thread is in the middle of another task, the response is delayed until the main thread is free. In most cases, all the work of rendering a website, running JavaScript and responding to user input happens on the main thread. It’s useful for understanding how long is a page unable to respond to user input during loading. Total Blocking Time (TBT) describes the JavaScript main thread activity. To learn more about how the score will be calculated and what’s changed from the previous version, check out the performance score calculator. In Lighthouse (starting at version 6.0) LCP will be used for calculating the Performance score. Largest Contentful Paint is available in Calibre, Chrome DevTools and through the Largest Contentful Paint API. Experimenting with FMP provided valuable insights for developing LCP. FCP is still relevant because it gives users feedback that the page is actually loading. The fact that we now have LCP does not mean that other metrics were useless. easy to compute and report in RUM tools.Computationally intensive, so it’s not available for real-user monitoring (RUM) in any major browser. Tracks the visual progress of loading the content in the viewport and averages the "completeness" of the page at various points in time Not accurate in about 20% of the cases. Not standardized and difficult to implement across browsers. The time of the paint after the biggest layout change during page load Often not relevant to users (for example, loading indicators and navigation bars). The time when the browser renders the first piece of DOM content Thorough testing has proved that LCP is a good approximation of when the main content of a page loads. ![]() The largest element, for example, a big paragraph of text in an article or an image on a product page, is likely the most useful for understanding the content of a page. Largest Contentful Paint (LCP) measures when the largest content element becomes visible in the viewport. The result of this process are three new performance metrics that fill in the gaps in the user experience story: As understanding and ability to measure user experience evolve, the metrics need to evolve as well. These metrics brought us a step closer to measuring user experience, but none of them are perfect and the industry keeps researching and developing key indicators for what describes a good user experience. First Paint (FP), First Contentful Paint (FCP)įirst Meaningful Paint (FMP), Speed Index (SI) ![]()
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