LargestContentfulPaint: renderTime property

The renderTime property can have the following values:

• A timestamp representing the time in milliseconds that the element was rendered to the screen.
• 0 if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

This example uses a PerformanceObserver notifying of new largest-contentful-paint performance entries as they are recorded in the browser's performance timeline. The buffered option is used to access entries from before the observer creation.

const observer = new PerformanceObserver((list) => {
  const entries = list.getEntries();
  const lastEntry = entries[entries.length - 1]; // Use the latest LCP candidate
  console.log(lastEntry.renderTime);
});
observer.observe({ type: "largest-contentful-paint", buffered: true });

For security reasons, the value of the renderTime property is 0 if the resource is a cross-origin request. Instead the loadTime is exposed. To expose cross-origin render time information, the Timing-Allow-Origin HTTP response header needs to be set.

For example, to allow https://developer.mozilla.org to see renderTime, the cross-origin resource should send:

Timing-Allow-Origin: https://developer.mozilla.org

Alternatively, you can use startTime which returns the value of the entry's renderTime if it is not 0, and otherwise the value of this entry's loadTime. However, it is recommended to set the Timing-Allow-Origin header so that the metrics will be more accurate.

If you use startTime, you can flag any inaccuracies by checking if renderTime was used:

const isAccurateLCP = entry.renderTime ? true : false;