$ time docker save nicolaw/tiddlywiki \
    | xz -z -9 -v -c -e -T 0 > nicolaw-tiddlywiki.img.xz
  100 %        27.8 MiB / 104.4 MiB = 0.266   1.9 MiB/s       0:53             

real	0m53.812s
user	0m52.374s
sys	0m0.478s

$ ls -alh nicolaw-tiddlywiki.img*
-rw-r--r-- 1 nicolaw nicolaw 105M Jun 17 01:09 nicolaw-tiddlywiki.img
-rw-r--r-- 1 nicolaw nicolaw  28M Jun 17 01:15 nicolaw-tiddlywiki.img.xz

$ time curl -sSL file://$PWD/nicolaw-tiddlywiki.img.xz | docker load
Loaded image: nicolaw/tiddlywiki:latest

real	0m1.708s
user	0m0.077s
sys	0m0.059s