The usual culprits are a slow server response, render-blocking CSS and JavaScript, uncompressed or unsized images, and fonts that delay text painting. The fastest wins are almost always: serve modern image formats at the right dimensions, preload the LCP element, and stop shipping blocking third-party scripts above the fold.
Measure in the field, not the lab. A Lighthouse run from a developer laptop routinely reports half the LCP that the Chrome User Experience Report shows for real visitors on mid-range phones and mobile networks. Field data is the number Google uses and the number your customers lived.