LargestContentfulPaint: renderTime property

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

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 or a coarsened timestamp if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

Cross-origin image render time

For security reasons, the value of the renderTime property was originally 0 if the resource is a cross-origin request.

Browsers may now expose a slightly coarsened render time in these situations. Check for browser support.

To expose more accurate 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 an accurate renderTime, the cross-origin resource should send:

http

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

Use `startTime` over `renderTime`

Regardless of the accuracy of the renderTime, developers should use startTime over renderTime as the LCP time. This returns the value of the entry's renderTime if it is not 0, and otherwise the value of this entry's loadTime so removes the need to check for 0 values for non-supporting browsers.

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.

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 });