Webvitals.js Impact on Core Web Vitals

new PerformanceObserver((list) => { console.log(list.getEntries()); 

}).observe({type: 'layout-shift', buffered: true}); 

import {onCLS} from 'web-vitals';

// Measure and log CLS in all situations
// where it needs to be reported.
onCLS(console.log);

• Only add up more score if the layout-shift event occurred less than 1 second after the previous entry and less than 5 seconds after the first entry in the session
• Do not report if the page is loaded from the BackForward (BF) cache and the persisted flag is true
• Do not report if the FCP is not captured
• Do not report if the page was loaded in the background
• Do not report if the CLS score is higher than 100

• Reported again (with the above exceptions) after a page is restored from the back/forward cache.
• Not reported if the page was loaded in the background.
• Should be reported any time the page's visibilityState changes to hidden.

• Data is supported in modern Chromium-based browsers, including Google Chrome, Microsoft Edge, and Opera.
• Can only be recorded whenever a user interaction occurs

• Reported again (with the above exceptions) after a page is restored from the back/forward cache.
• Not reported if the page was loaded in the background
• Should be reported any time the page's visibilityState changes to hidden.
• Not reported if the page was loaded, but the user never clicked, tapped, or pressed a key on their keyboard.
• Not reported if the page loaded, but the user interacted with it using gestures that aren't measured, such as scrolling or hovering over elements.
• Not reported if the page is being accessed by a bot such as a search crawler or headless browser that has not been scripted to interact with the page.

Methodology is the same. See this article from web.dev for more information. 

Captured directly from the library.

import {onLCP} from 'web-vitals';

// Measure and log LCP as soon as it's available.
onLCP(console.log);

• If no LCP events were fired
• If the page view is not a full page turn (e.g it’s SPA or a custom timer)

• Reported again (with the above exceptions) after a page is restored from the back/forward cache.
• Not reported if the page was loaded in the background.