Core Web Vitals

CLS measures the visual stability of the content on a page by counting the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the entire lifespan of the page. A layout shift happens when an element changes from its starting position due to resources loading later in a page's lifecycle and pushing content below it down the page. For example, a user is online shopping and right as they try to click the add to cart button an ad loads above the button and pushes it down. This layout shift causes the user to accidentally click the ad instead of the add to cart button. A low CLS can help ensure your users have a positive experience on your site and can quickly accomplish what they came to do.

FID measures the delay in interactivity and responsiveness. This is the time from when the user interacts with your content by clicking a link, tapping a button, or pressing a key to when the browser processes that interaction. FID only counts the delay, not the time it takes to process the event or how long it takes the UI to update. A delay typically happens because the browser is busy loading the rest of the page content and cannot respond to the user’s interaction yet. A short FID can help increase the chance that the user remains on the page and interacts with your content. Currently, most websites perform well on FID measurement.

LCP measures the time it takes for the largest visible element on the page to load relative to when the page first started loading. As different elements load, the LCP can change. The browser stops measuring LCP when the user begins interacting with the page, since interactions typically change what is visible to the user. A fast load speed can reassure the user that the page is useful because they are able to quickly begin viewing content.

Elements that can be the LCP:

INP refers to how the page responds to specific user interactions that are programmed into the overall INP metric measured by Google Chrome’s lab data and field data. INP is designed to provide a representation of the overall interaction delay of a page. It does this by working from a sample of the single longest interactions that happen when a user visits the page. For example, if a page has less than 50 total interactions, INP takes into consideration the interaction that has the absolute worst delay. The measurement of INP is a representation of how long a user has to wait in order to interact with the entire page. This is a direct contrast to FID (First Input Delay).

INP is concerned with the following types of interactions:

Google’s web.dev documentation explains that a good INP value is around 200 milliseconds or less.

TTFB measures the length of time that it takes for the first byte to be transferred by the server.

TTFB has long been suspected as a driver of significant performance gains, which means that it’s a priority that SEO professionals should be optimizing as part of their SEO process.