A .tar archive is basically only the files cat’ed together, with a header and index added, right?
And a .tar.gz takes forever to modify, because it needs to first extract the .tar.
So why is there no archive format that just cat’es the compressed files together?
I mean… you can do that manually. Apply gzip or xz or whatever to each file then just make a tarball of the compressed files. You could write a shell script to do that.