LCP measures how long the largest visible element takes to render. Good is under 2.5 seconds; the usual culprits when it exceeds that are unoptimised hero images, render-blocking JavaScript, and slow server response.
INP (which replaced FID in 2024) measures the worst-case responsiveness of any user interaction over the page's lifetime. Good is under 200ms. Heavy main-thread JavaScript is almost always the cause when this fails.
CLS measures unexpected layout shifts after the page has loaded. Good is under 0.1. The most common offenders are images without width/height attributes, ads injecting above existing content, and web fonts swapping in without a `font-display: swap` fallback.
Core Web Vitals are a confirmed (small) ranking factor, but their bigger impact is on actual conversion: every additional second of load time measurably reduces purchase rate, sign-up rate, and time on page.