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

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https://bugs.launchpad.net/bugs/1701132

Title:
  Please use xz instead of pxz

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