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 -
kekaxz seems to just run with 2 threads
This is curious. Checking I'm seeing the more compression is applied (-1 to -9) the less threads are used.
Due to how Keka works (NSTask) and how bsdtar's --use-compression-program works (needs -d and -c arguments, 7z uses x/e and a instead) I'm unable to directly do a tarball using 7Z. I'll investigate a bit more.
But I tested the xz binary from homebrew with
brew installs 5.3.1 alpha version of XZ, maybe this is fixed in that version. Keka uses latest stable 5.2.5 version. Not sure if using an alpha version is a good idea for an stable Keka version.