Google has announced an encoder which is claimed to reduce JPEG file size substantially while maintaining image quality:
https://research.googleblog.com/2017...urce-jpeg.html
I've had a bit of a play with it and I'm impressed so far. It took a high res mosaic down from 16.3MB to 11.6MB and still looked great even when pixel peeping. It did take a long time, though
Might be a way to improve IQ when file size is limited, e.g. IIS image attachments.
Cheers,
Rick.