Comment 5 for bug 1701132

The original command's CPU usage is the following in xenial:
rbalint@xenial-test:~/Downloads$ time pxz -T4 -c ubuntu-16.04.3-preinstalled-server-armhf+raspi2.img > ubuntu-16.04.3-preinstalled-server-armhf+raspi2.img.p.xz

real 3m31.117s
user 10m12.812s
sys 0m17.988s

rbalint@xenial-test:~/Downloads$ ls -al ubuntu-16.04.3-preinstalled-server-armhf+raspi2.img.p.xz
-rw-rw-r-- 1 rbalint rbalint 254621780 Nov 16 00:03 ubuntu-16.04.3-preinstalled-server-armhf+raspi2.img.p.xz

On the same system I retested the matrix of xz options and it shows that -T is ignored and -3 has the closest wall clock time with 258680 kB result which is slightly larger than the original but even -8 consumes system time and creates smaller compressed image:

rbalint@xenial-test:~/Downloads/xz-test$ 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 82.59 74.28 1.51 4792 25.38 25.16 0.10 2404 329984
-1 -T1 109.10 96.64 1.63 10712 23.63 22.55 0.08 3080 298056
-2 -T1 136.69 127.53 1.78 18392 22.36 21.71 0.08 4196 286276
-3 -T1 164.91 158.57 1.64 33948 21.46 20.93 0.09 6260 275036
-4 -T1 320.00 314.05 1.90 50320 21.18 20.86 0.13 6120 258680
-5 -T1 375.79 370.80 1.82 97480 20.46 20.23 0.10 10340 247316
-6 -T1 476.02 471.64 2.06 97416 20.45 20.18 0.10 10352 245768
-7 -T1 497.44 492.16 2.35 191624 21.24 20.06 0.09 18408 240188
-8 -T1 513.29 507.79 2.24 380108 19.43 19.16 0.10 34908 229736
-9 -T1 873.02 579.58 7.55 690952 22.61 19.30 0.14 67700 224492
-0e -T1 746.35 743.14 1.76 6224 23.69 23.32 0.11 2420 301004
-1e -T1 809.93 806.56 1.98 14988 21.57 21.31 0.11 3180 272976
-2e -T1 846.12 843.60 1.82 26748 22.10 20.75 0.07 4140 262860
-3e -T1 699.59 696.83 1.97 50316 20.90 20.68 0.07 6260 252052
-4e -T1 879.00 875.42 2.04 50256 20.35 19.96 0.10 6292 252048
-5e -T1 723.80 720.15 1.81 97480 21.35 20.37 0.11 10360 245172
-6e -T1 911.69 908.11 2.03 97464 20.37 19.83 0.07 10348 245168
-7e -T1 942.32 936.49 2.09 191616 19.72 19.25 0.12 18352 239552
-8e -T1 967.20 962.26 2.42 380156 18.85 18.07 0.14 34904 228956
-9e -T1 1177.95 1036.56 6.36 690784 19.78 18.08 0.11 67680 223748
-0 -T4 81.56 73.64 1.46 4820 29.27 25.82 0.09 2252 329984
-1 -T4 103.68 95.14 1.67 10800 24.41 22.86 0.12 3180 298060
-2 -T4 134.05 126.22 1.76 18480 22.06 21.65 0.13 4232 286276
-3 -T4 170.57 162.72 1.72 33812 21.62 20.94 0.12 6188 275036
-4 -T4 322.15 317.83 2.10 50284 21.55 21.05 0.07 6136 258684
-5 -T4 407.15 399.93 1.92 97656 21.85 20.50 0.12 10412 247320
-6 -T4 478.64 473.32 1.95 97660 21.92 20.34 0.09 10348 245772
-7 -T4 493.97 485.52 1.99 191692 20.66 20.01 0.08 18548 240192
-8 -T4 512.28 506.08 2.20 380096 22.03 19.58 0.11 34888 229740
-9 -T4 652.71 559.20 4.78 691076 20.57 19.15 0.08 67700 224496
-0e -T4 750.00 743.68 1.74 6172 24.59 23.34 0.11 2444 301004
-1e -T4 812.97 805.57 1.94 15004 21.81 21.36 0.11 3280 272976
-2e -T4 849.79 842.25 2.05 26828 20.96 20.63 0.12 4312 262860
-3e -T4 702.49 696.67 1.69 50380 20.85 20.61 0.09 6360 252056
-4e -T4 882.09 874.62 1.97 50552 20.11 19.89 0.08 6292 252052
-5e -T4 725.29 719.12 1.90 97512 20.42 20.20 0.06 10320 245176
-6e -T4 917.31 908.11 1.87 97632 20.26 19.78 0.12 10312 245168
-7e -T4 941.70 935.85 1.91 191656 20.47 19.27 0.10 18432 239552
-8e -T4 966.09 961.14 2.21 380140 18.52 18.10 0.13 35028 228956
-9e -T4 1147.79 1040.24 5.46 690956 19.88 18.09 0.06 67612 223748