I’ve got a very slow pristine-tar commit operation I could speed
up by telling pristine-tar that it was produced with, for example,
-Hustar -b1 --numeric-owner --owner=0 --group=0 --mode='u=rwX,go=rX'
with GNU tar, followed by xz -7e (specifically because this was
created from a repack script). Or paxtar -Mdist, or whatever.

This way, it, at the *very* least, wouldn’t have to try all xz
combinations trying to figure out the compression level, and
could maybe do a few more sensitive things with the ustar archive.