Google Guetzli - new JPEG image encoder by Google
They don't exaggerate when they say it's extremely
slow at the moment. The compression of the included loco.png took like 2 minutes on my i7-6700K / 32GB RAM.
A 24Mpix image would probably take ages to compress.
Do pixelpeep. The quality should be
30% to 40% better. Is it really? The JPEG artifacts are definitely less visible - that's great. On the other hand, there some stripes and squares on the sky introduced which I don't like too much.
(Feel free to compress the loco.png image to a similar size with your favourite software and include here for comparison.)