Important metrics to measure

• First Contentful Paint (FCP)
• Largest Contentful Paint (LCP)
• First Input Delay (FID)
• Time to Interactive (TTI)
• Total Blocking Time (TBT)
• Cumulative Layout Shift (CLS)

LCP

new PerformanceObserver((entryList) => {
  for (const entry of entryList.getEntries()) {
    console.log('LCP candidate:', entry.startTime, entry);
  }
}).observe({type: 'largest-contentful-paint', buffered: true});

web-vitals

https://github.com/GoogleChrome/web-vitals

import {getLCP, getFID, getCLS} from 'web-vitals';
getCLS(console.log);
getFID(console.log);
getLCP(console.log);

TTI

FID

First input delay

new PerformanceObserver((entryList) => {
  for (const entry of entryList.getEntries()) {
    const delay = entry.processingStart - entry.startTime;
    console.log('FID candidate:', delay, entry);
  }
}).observe({type: 'first-input', buffered: true});