Largest Contentful Paint is the metric clients recognise from PageSpeed screenshots. Monitoring it only helps if you know which clock you are reading and which template you pinned.
What LCP is measuring
LCP marks when the largest content element in the viewport finishes rendering. On ecommerce and content sites that is often a hero image, a heading block, or a product image. Lab tools estimate it under controlled conditions. CrUX reports how real Chrome users experienced it on the origin or URL, with delay and eligibility rules that confuse teams who expect yesterday’s deploy to appear tomorrow.
Lab vs field for LCP
A green Lighthouse LCP with amber CrUX usually means your lab device and network are kinder than the field, or that the field sample includes templates you never test. A red lab LCP with green CrUX can mean you are stressing a cold cache path users rarely hit. Agencies that paste one homepage into GTmetrix and declare victory skip that distinction.
Scheduled monitoring keeps a dated lab series on the templates you chose. When PSI returns CrUX, you can show field context without pretending the two numbers are interchangeable.
What to pin and what to alert
Pin money templates: home, primary listing, product or article, and checkout or lead form if they exist. Set an LCP budget you can defend (many teams start from Google’s “good” threshold and tighten for key pages). Alert when lab LCP crosses the budget on a monitored URL after a deploy window, then use a diagnostic tool for the waterfall if you need element-level proof.