Comment 4 for bug 1732681

In development branch xz -0 is used for compressing the Ubuntu Core image as well but xz's compression setting have little impact on its compressed size thus -0 can be kept there:

(ubuntu-core-16-amd64.img)
ubuntu@bb:~/test-core$ cat time-size.txt
Compr. Threads Wall.t User.t Sys.t Res.t Wall.t User.t Sys.t Res.t Size(kB)
-0 -T1 79.97 76.04 1.54 5068 10.37 6.82 0.08 2528 381472
-1 -T1 119.13 115.42 1.76 10952 11.65 6.90 0.10 3376 379540
-2 -T1 148.43 142.16 1.74 18612 11.83 6.92 0.11 4440 379504
-3 -T1 164.89 158.90 1.93 34076 10.05 6.88 0.14 6416 379496
-4 -T1 178.44 172.44 1.85 50616 11.58 6.91 0.13 6296 379464
-5 -T1 187.43 181.44 2.13 97644 17.77 7.21 0.10 10484 379444
-6 -T1 201.22 194.96 2.03 97716 14.27 8.59 0.15 10572 379444
-7 -T1 201.56 194.90 1.97 191952 14.57 8.53 0.15 18660 379444
-8 -T1 200.86 195.67 2.10 380228 13.11 8.61 0.10 35112 379440
-9 -T1 283.20 238.30 4.75 689920 16.62 8.59 0.13 68004 379440
-0 -T4 33.40 109.76 2.43 24920 12.12 8.47 0.09 2744 381844
-1 -T4 56.35 174.24 2.91 73592 11.99 8.51 0.10 3296 379668
-2 -T4 55.58 185.15 3.82 141184 12.63 8.55 0.10 4376 379572
-3 -T4 55.92 194.41 3.15 276356 11.82 8.67 0.10 6348 379528
-4 -T4 63.59 218.39 3.29 342144 11.74 8.57 0.09 6416 379496
-5 -T4 64.24 226.16 5.84 675728 12.31 8.56 0.12 10488 379460
-6 -T4 67.20 239.20 5.55 675512 12.88 8.54 0.15 10652 379456
-7 -T4 3539.73 306.44 98.61 740100 12.55 8.60 0.12 18632 379448
Command terminated by signal 2
-8 -T4 34404.61 126.70 410.87 720576