It is almost always a hero image, a heading or a video poster, and it is the metric most directly tied to whether somebody stays. The useful property is that it points at one specific element, so the investigation has a subject: find out which element it is, then find out why that element is late.
The causes are a short list. The image is too large or in the wrong format; it is discovered late because it is behind JavaScript or a CSS background rather than in the HTML; a font or a script is blocking the render; or the server took too long to send the first byte. Almost every real LCP problem is one of those four, and the order they are checked in should follow that.
Related terms
Core Web Vitals
Google's three field metrics for loading, responsiveness and visual stability, measured on real visits rather than in a lab.
Static generation (SSG)
Rendering pages to HTML at build time, so a request is answered by handing over a file instead of running code.
Hydration
The step where JavaScript takes over server-rendered HTML in the browser and makes it interactive.
The bench this belongs to
Web performanceA rebuild that loads on a phone on mobile data, in the country your customers are actually in. Core Web Vitals are the measurement, revenue is the point.
