Compressing tar.xz uses kekaxz and it only seems to be using 2 threads and one core... use 7z with the -txz option instead

Looking at the Activity Monitor, kekaxz seems to be used if you have "Always tarbal..." checked in the Preferences instead of the 7z... and for some reason, kekaxz seems to just run with 2 threads and no more than 100% CPU, which would be a single core.

I can't test the kekaxz in the Terminal if it's sandboxed and refuses commands...

But I tested the xz binary from homebrew with
`tar -cf - 1.psd | /usr/local/Cellar/xz/5.2.5/bin/xz -9 -T8 > 1.psd.tar.xz`
and that uses more than just two threads and more than just 100% CPU (one core) if -T8 is specified... if it is omitted, then it just seems to use a single thread and 100% CPU

I suggested using 7z for it before, since 7z has an option -txz for .xz format and as it was mentioned there, 7z generally seems to have been using more threads and that it seemed faster than using xz
https://github.com/aonez/Keka/issues/145

This works fine, even if the number of threads isn't specified and it generally seems to be using more CPU % than xz does
`tar -cf - 1.psd | 7z a -si -txz -mx=9 1.psd.tar.xz`

And to uncompress it, you can just use this
`7z e -so 1.psd.tar.xz | tar -x -`