Largest Contentful Paint (LCP) is a new page load metric available as a performance API, which describes page speed as the speed of delivering the largest contentful element to the screen. It is the high-level metric (where Element Timing is its low-level primitive) aiming to provide a meaningful result even for developers who don't annotate their sites.
Developers today don't have a reliable metric that correlated with their user's visual rendering experience. Existing metrics such as First Paint and First Contentful Paint focus on initial rendering, but don't take into account the importance of the painted content, and therefore may indicate times in which the user still does not consider the page useful. Largest Contentful Paint (LCP) aims to better correlate with user experience than the existing page-load metrics, in a way that is easy to understand and reason about and reduces the chance of being gamed.
Docs: https://github.com/WICG/LargestContentfulPaint
Explainers: https://github.com/WICG/LargestContentfulPaint