In srcset it often happens that original image is referenced when it's smaller than "2x" the thumb size.
But the originally uploaded image is unneccessarily big in filesize: It has not passed ImageMagick, thus it contains useless metadata and may have an unwanted high quality. I use jpeg compression of 69%. Most of all the originals are delivered too big.
That's even more f***ed since srcset is used for mobile devices where bandwidth is low.
Solution: Pass originals through the thumb process and call these in content display.
The real originals should only be linked (not embedded) on the image page.