Today, is a great day! WordPress 4.4 was released and it now creates by itself the src-set tag, including all your image sizes. That enabled the browser to load the most appropriate size for your browser device :)
Now, WordPress still doesn’t generate the Retina (high-dpi) images and WP Retina 2x will do its duty, as it always did. If you have the latest version of WP Retina 2x, it already adds the retina images directly in the src-set created by WordPress.
I recommend you to have a new look at the settings of WP Retina 2x. Thanks to WordPress 4.4, you optimize the retina process and make the whole solution really neat.
Of course, keep the Auto Generate, you need your Retina Images. You also need to keep the Responsive part of WP 4.4 but you have this option to disable if you don’t like it (or have issues with it). I recommend to also disable the new “Medium Large” size that was added by 4.4, it’s not neat, it’s just an alternative size just in case. Since you are using WP Retina 2x, all the sizes are perfect, you don’t need this un-perfect size in the middle.
A new “Responsive Images” method has been added. This method simply adds the retina images in the src-set created by WP 4.4. When all the plugins and theme support WP 4.4 well, this method will be the most optimized one. Until then, keep on using the current recommended method, PictureFill. It does what the “Responsive Images” is doing but also rewrite the HTML for the older IMG tags to include the retina images.
If you have any questions, please ask below. Enjoy those new releases, towards a cleaner, faster WordPress :)