Cumulative Layout Shift – layout shifts are reported when an element that is visible within the viewport changes its start position between two frames. A good CLS score is less than 0.1.
First Input Delay – the time between when a user attempts to interact with a page, for example, by clicking a link, and when the browser starts processing event handlers in response to the request. A good FID score is under 100 ms.
Largest Contentful Paint – the time it takes for a page’s largest image or text block to render relative to when the page first loads. A good LCP score is under 2.5 seconds.
The DOM (Document Object Model) defines the logical structure of documents, and the DOMContentLoaded fires when all the nodes in the page have been constructed in the DOM tree without waiting for images and CSS to finish loading.
When an object has been loaded. Onload is most often used within the element to execute a script once a web page has completely loaded all content such as images and iframe.
Time To First Byte – the time between the request for a resource and when the first byte of a response begins to arrive. Although not considered a core web vital, it is a foundational metric that precedes every other loading metric for navigation requests, including LCP. A good TTFB score is under 800 ms.