LargestContentfulPaint.renderTime

let renderTime = LargestContentfulPaint.renderTime;

try {
  let lcp;

  const po = new PerformanceObserver((entryList) => {
    const entries = entryList.getEntries();
    const lastEntry = entries[entries.length - 1];
    console.log(lastEntry.renderTime);
  });

  po.observe({type: 'largest-contentful-paint', buffered: true});

} catch (e) {
  // Do nothing if the browser doesn't support this API.
}