If compressing with pxz use more threads than the
default of 4.

Going from 4 to 8 is almost linear.  Going from 8 to
16 is about 50%. Beyond 16 starts to see diminishing returns,
however because this was tested on a 16 core machine it may
be that going past physical cores is the tipping point.

I plan to revisit this and if that is indeed the case have it autoscale
to the number of physical cores, or something more intelligent than just
picking a value.

However since the current system is simply picking a value (4)
I see no harm in increasing it as a first pass, especially when it
speeds up the compressing images steps by over a minute each.