The height/width attributes aren’t enough, the image would need to be resized appropriately. The HTML attributes just constrain the size on the rendered page, doesn’t affect the download size.

@mbabker I agree, it doesn’t affect the download size

The reason I'm reporting this is because:
Specifying a width and height for all images allows for faster rendering by eliminating the need for unnecessary reflows and repaints.

Details from Google:
When the browser lays out the page, it needs to be able to flow around replaceable elements such as images. It can begin to render a page even before images are downloaded, provided that it knows the dimensions to wrap non-replaceable elements around. If no dimensions are specified in the containing document, or if the dimensions specified don't match those of the actual images, the browser will require a reflow and repaint once the images are downloaded. To prevent reflows, specify the width and height of all images, either in the HTML tag, or in CSS.

Thank you for raising this issue.

Joomla 3 is now in security only mode with no further bug fixes or new features.

As this issue doesn't relate to Joomla 4 it will now been closed.

If we are mistaken and this does apply to Joomla 4 please open a new issue (and reference this one if you wish) with updated details for testing in Joomla 4.
cc @zero-24