There's definitely a performance issue! Not least in that you have to wait for layout to be calculated before you can even start fetching the image you need to render. That's why the sizes attribute exists, so the browser can start downloading the correctly sized image straight away. It's not impossible to account for in a CMS but it's a lot of work - essentially the image tag's markup (sizes attribute) needs to vary based on its container(s) and what we know about how those containers will constrain the image's width.