With todays phones and network connections, downloading and rendering images actually constitutes only a small proportion of the total loading time.

Of that small proportion, the cpu time of rendering the images, relayout of the page for changed image dimensions, and re-rendering the image and all overlapping text and layers each time a bit more of it partially loads, is more substantial than the network delay of loading the data.

The TL;DR: is that optimising images isn't the place to start to make most websites load faster, especially on mobile.

Obviously if CPU speeds increase faster than network speeds, image size may matter again in the future. But today the size is mostly irrelevant as long as you aren't serving your photographs as multi-megapixel images or gifs.