I too also agree with the above.
To me the best solution is serving images of different sizes and resolutions based on device and connection speed, this way the customers should get a balance between speed and quality.
E.g. serve a x2 res image on retina or retina equivalent if the speed dictates that this is an option. alternatively, if it is a slow device connection, say via limited broadband speed, serve a lower resolution/file size to ensure at least something is delivered in an optimised speed.
I would suggest playing and finding the right balance for your site and market.
Finally, I find that using SVG images is the best for gaining a speed advantage.
Cheers
Tim