PerformanceElementTiming: element プロパティ

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

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