Largest Contentful Paint or LCP is a landmark timing based metric that reports the time at which the largest above-the-fold element was rendered.
To calculate the largest element, LCP considers the following:
In Calibre, LCP is sourced directly from Chrome browser. The browser emits this timing as part of it's tracing.
For more information regarding how Largest Contentful Paint can be used, read this guide by Philip Walton on web.dev.