Brunsli: Practical JPEG repacker (now part of JPEG XL)
Preemptive disclaimer: I don't want to belittle the work the authors did here in any way, and are actually excited especially about the reversible, lossless jpeg<->brunsli coding and that google's buy in and network effects will most likely mean this comes to a viewer/editing software near you in the not so distant future (unlike lepton, which never got out of it's tiny niche).<p>But seeing the 22% improvement figure reminded me that the typical JPEG file on the internet is rather unoptimized even on write-once-read-many services like imgur or i.reddit.com which transform files (stripping meta data etc) and do not preserve the original files. 
Just using the regular vanilla libjpeg encoder you can usually save 5%-10% just by lossless recoding of the coeffs and the better-yet-more-computionally-intense mozjpeg coder can even get you a bit further than that.<p>Then again, the imgur single image view page (desktop) I just randomly opened by randomly clicking an item on their homepage transfers 2.9MiB of data with ads blocked (3.9MiB deflated), 385KiB of which was the actual image, and that image can be lossless recoded by mozjpeg to 361KiB (24KiB difference, a 6.2% reduction), so the 24KiB (0.8%) reduction out of 2.9MiB of cruft hardly matters to them I suppose and may be cheaper in bandwidth and storage cost to them than the compute cost (and humans writing and maintaining the code).<p>Using brunsli, that same 385KiB imgur file went down to 307KiB so roughly a 20% reduction, but still only 2.6% reduction of that massive 2.9MiB the imgur site transferred in total.