xz -T0

xz -v -T0 file_to_compress.tar
xz -T0

DIR="YOUR_DIR" bash -c 'tar c "$DIR" | xz -T0 -c > "$DIR".tar.xz && ls -lh "$DIR".tar.xz'

See also

• Data compression: LZMA, xz, unxz, xz -t, xz -q, xz -T0, xz -9, zstd, Igor Pavlov