The id read-only property of the PerformanceElementTiming interface returns the id of the associated element.

A string.

In this example an <img> element is being observed by adding the elementtiming attribute. A PerformanceObserver is registered to get all performance entries of type "element" and the buffered flag is used to access data from before observer creation. It will log myImage to the console, this being the id of the image element.

<img
  src="image.jpg"
  alt="a nice image"
  elementtiming="big-image"
  id="myImage" />

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    if (entry.identifier === "big-image") {
      console.log(entry.id);
    }
  });
});
observer.observe({ type: "element", buffered: true });