What Core Web Vitals measure
Largest Contentful Paint (LCP) is how fast the main content appears; good is 2.5 seconds or less. Interaction to Next Paint (INP) is how quickly the page responds to taps and clicks; good is 200 milliseconds or less. Cumulative Layout Shift (CLS) is how much the layout jumps around; good is 0.1 or less.
Lighthouse can't measure INP in a lab test, so Total Blocking Time (TBT) stands in for it: heavy JavaScript that blocks the main thread usually hurts both.
Why fix order matters
Most slow sites have the same few causes: oversized images, render-blocking CSS and JavaScript, too many third-party scripts and missing image dimensions. Fixing the largest one first, usually the LCP image, often moves the score more than ten small tweaks.
Lab data versus field data
The score at the top of PageSpeed Insights is a lab test on a simulated phone. Google ranks on field data from real Chrome users, shown in the 'Discover what your real users are experiencing' section. Both matter, but the field data is what Google uses.