Great question. I have the same issue. Have you found a work around?
In my case, I have featured images cut in Photoshop to 1038x576px, per best practices. When I import an image, WP creates an jpeg of 672x372 to go into the featured image section on the homepage. WP correctly adds width="672" height="372" attributes, so far so good.
But then WP scales the image to 341x189, and GTMetrix flags the site for serving scaled images.
I am willing to cut separate images in Photoshop to 341x189 to be used specifically as featured images on the homepage, but is there any way to swap my "cut to size" featured images so they appear only on the homepage? And can I manually set the width and height attributes to 341 and 189, respectively?
I have a child theme. Any suggestions or insights would be appreciates.