The naturalWidth read-only property of the PerformanceElementTiming interface returns the intrinsic width of the image element.

An unsigned 32-bit integer (unsigned long) which is the intrinsic width of the image if this is applied to an image, 0 for text.

In this example an <image> 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. The image file has a width of 1000px and a height of 750px. Calling entry.naturalWidth returns 1000, that being the intrinsic width in pixels.

<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.naturalWidth);
    }
  });
});
observer.observe({ type: "element", buffered: true });