Wordpress 4.4 has a new feature called Responsive Images Basicly that find all tags in the output just before it is send to the browser and decorate all tags with srcset & sizes attributes so the browser can determine which image to download. This is an enormous performance improvement for mobile/tablit devices since they now can download the image which best suits the mobile devices resolution

see http://wptavern.com/wordpress-4-4-clifford-featuring-responsive-images-twenty-sixteen-and-more

I'm hacking around on something like this. I hope to release a first version this weekend.

I'll post status updates here.

See https://octobercms.com/forum/post/responsiveimages-plugin