cjwatson@pepo:~$ for x in 0 1 2 3 4 5 6; do time xz -${x}c Packages | wc -c; done
9054452

real    0m2.397s
user    0m2.388s
sys     0m0.004s
8511152

real    0m2.920s
user    0m2.900s
sys     0m0.008s
8248444

real    0m4.099s
user    0m4.060s
sys     0m0.028s
8096632

real    0m7.157s
user    0m7.020s
sys     0m0.108s
7390432

real    0m10.098s
user    0m10.021s
sys     0m0.052s
7043520

real    0m15.030s
user    0m14.937s
sys     0m0.048s
6885052

real    0m18.889s
user    0m18.757s
sys     0m0.064s