tl;dr: xz offers much better compression but at the expense of far slower (de)compress speed. we should probably switch to gz (or some other codec offering a better balance of compression/decompression speeds), but it's hard to confirm the overall improvement without first rebuilding world.
Reference benchmark
https://www.opencpu.org/posts/brotli-benchmarks/
Tested below packages by running hab pkg install core/<pkg> inside a studio and then tarring up the contents of /hab/pkgs/core/<pkg>
$ ls -lh *.tar
-rw-r--r-- 1 chetan staff 351M Sep 27 11:26 jdk8.tar
$ tar -tf jdk8.tar | wc -l
1846
$ time xz --keep jdk8.tar
real 2m25.787s
$ time gzip --keep jdk8.tar
real 0m13.055s
$ ls -lh
-rw-r--r-- 1 chetan staff 173M Sep 27 11:26 jdk8.tar.gz
-rw-r--r-- 1 chetan staff 143M Sep 27 11:26 jdk8.tar.xz
$ time gunzip --keep jdk8.tar.gz
real 0m1.480s
$ time xz -d --keep jdk8.tar.xz
real 0m8.950s
$ ls -lh mysql.tar
-rw-r--r-- 1 chetan staff 1.7G Sep 27 11:58 mysql.tar
$ tar -tf mysql.tar | wc -l
13732
$ time gzip --keep mysql.tar
real 1m5.738s
$ time xz --keep mysql.tar
real 10m58.610s
-rw-r--r-- 1 chetan staff 439M Sep 27 11:58 mysql.tar.gz
-rw-r--r-- 1 chetan staff 240M Sep 27 11:58 mysql.tar.xz
$ time gunzip --keep mysql.tar.gz
real 0m7.213s
$ time xz -d --keep mysql.tar.xz
real 0m28.616s