By default xz compressor uses single thread (eg. to compress newly created packages from AUR). There's a --threads option for using more threads.
Where can I set global settings for xz so that threads option is set to my value? Could'nt find any info in man.
The man page does talk about the use of environment variables for this purpose:
ENVIRONMENT
xz parses space-separated lists of options from the environment variables XZ_DEFAULTS
and XZ_OPT, in this order, before parsing the options from the command line. Note
that only options are parsed from the environment variables; all non-options are
silently ignored. Parsing is done with getopt_long(3) which is used also for the
command line arguments.
XZ_DEFAULTS
User-specific or system-wide default options. Typically this is set in a
shell initialization script to enable xz's memory usage limiter by default.
Excluding shell initialization scripts and similar special cases, scripts must
never set or unset XZ_DEFAULTS.
XZ_OPT This is for passing options to xz when it is not possible to set the options
directly on the xz command line. This is the case e.g. when xz is run by a
script or tool, e.g. GNU tar(1):
XZ_OPT=-2v tar caf foo.tar.xz foo
Scripts may use XZ_OPT e.g. to set script-specific default compression op‐
tions. It is still recommended to allow users to override XZ_OPT if that is
reasonable, e.g. in sh(1) scripts one may use something like this:
XZ_OPT=${XZ_OPT-"-7e"}
export XZ_OPT
It looks like XZ_DEFAULTS will do what you want.