Now Google Insigth has a new metric called Cumulative Layout Shift (CLS) that relies in changes in the layout of the rendered page when a element changes in the viewport.
Whit lazyloaded images this occurs, and the solution is to put sizes in the parent div container of the image, so the space is “reserved” to the real image when is loaded.
But the parent div cointainer may not know what the image size is, so we need a catch-all size. This may be acomplised with a width: fit-content css property.
For example, you can view it in the url have posted. In the <div class="featured-image page-header-image-single ">
I have added the css property with a function in the functions.php of the child theme