Starting with case studies done by Google and other companies like NDTV have shown a strong correlation between Largest Contentful Paint (LCP) and Bounce rates, even sales. Thinking about it, if a page takes a while to load, users will leave less conversion/sales.
NDTV reduced their LCP by 55% and say a reduction of 50% in bounce rates.
There are a few tools available out there, some with price tags. Free Tools like those on Chrome, the web vitals extensions and the Chrome DevTools can track Web Vitals. One of the most common tools used is PageSpeed Insights. This tool provides both Field - If the website gains enough traction - and Lab data. Aside from the ability to test when the LCP is recorded, PageSpeed Insights also provides opportunities to improve FCP, LCP and much more.
Make a closer look at the assets and trim them out. Reduce unused CSS & JS
Address Layout Shift issues that could affect the rendering of the hero asset.
Create critical assets and preload them, allowing content above the fold render and becomes interactive first.