Three different measurements
Core Web Vitals include LCP, INP and CLS. LCP measures the time until the largest visible content element is rendered. INP measures responsiveness to user interactions, while CLS measures unexpected layout shifts. They do not measure the same phenomenon. [1]
Thresholds and visitor data
web.dev defines a good LCP as at most 2.5 seconds, INP as at most 200 milliseconds and CLS as at most 0.1. Assessment uses the 75th percentile of page visits, separately for mobile and desktop. A single laboratory test does not represent this distribution among real visitors. [1]
Image sizes for the display
The srcset and sizes attributes let the browser select an image variant appropriate to its display. The element width alone does not determine the required resolution: screen pixel density also matters. A display width of 600 pixels therefore does not always require an image exactly 600 pixels wide. [3]
Speed and Google
Google states that its ranking systems use Core Web Vitals, but good results do not guarantee top search rankings. Improvements should therefore be assessed using user experience measurements; one speed score cannot establish guaranteed sales or ranking growth. [2]