I think srcsets are a reasonably proxy for this. Serve extremely optimized images to mobile devices, and the full images to desktops.
It isn't perfect - you'll get some mobile devices on wifi that could have consumed the hero images and some desktop devices still on dial up, but it's still a lot better than doing nothing.
It isn't perfect - you'll get some mobile devices on wifi that could have consumed the hero images and some desktop devices still on dial up, but it's still a lot better than doing nothing.