Hey there,
maybe it would make sense to integrate mozjpeg Image optimizing for the output JPG-Tiles. It can save up to 10-50% of the inital JPG Filesize (depending on the Image) - without much impact on the Image itself.
Just did some tests on my Gigapixel Images and 360° Tours ...
Just a little idea for some future enhancement!
https://github.com/mozilla/mozjpeg
bye from sunny Tyrol
Andreas Schnederle-Wagner