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LargestContentfulPaint: renderTime property

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The renderTime read-only property of the LargestContentfulPaint interface represents the time that the element was rendered to the screen.

Value

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.

Examples

Logging the renderTime of the largest contentful paint

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.

js

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 });

Cross-origin image render time

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:

http

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:

js

const isAccurateLCP = entry.renderTime ? true : false;

Specifications

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
renderTime 77 79 No No 64 No 77 77 No 55 No 12.0

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https://developer.mozilla.org/en-US/docs/Web/API/LargestContentfulPaint/renderTime