this is a bit of a shot in the dark, but I believe we may have
unnecessary delta in our images caused by the rebuilding of software on
a daily basis. this would result in mtimes changing when the files
actually do not.
a tiny mtimer tool is meant to work around that by consuming an input
json file of mtimes+checksums and if mtimes change it will checksum the
affected file to verify it actually has changed in content as well.
assuming reproducible builds this should result in far less delta in the
erofs and by extension the delta download
