hadrian binary-dist is running slow (single-threaded) xz from gtar

I am doing hadrian binary-dist. The last step calls gtar, which calls xz, which uses only one thread, so it takes 8 minutes. Perhaps use tar -c -I "xz -e -T0"? (where T0 means: all cores)

NB: why do I make a bindist? It seems that's the only way to get make; sudo make install separation.