so you now have only one (old) machine where the issue is fairly reproducible...
from your last strace, supposedly in the old/slow machine, I'm seeing this last: write(1, "Freeing sfz file from memory...",... write(1, "OK",... write(1, "Stopping disk thread...",... write(1, "OK",...
is it really hanging or just taking longer to exit?
so you now have only one (old) machine where the issue is fairly reproducible...
from your last strace, supposedly in the old/slow machine, I'm seeing this last:
write(1, "Freeing sfz file from memory...",...write(1, "OK",...write(1, "Stopping disk thread...",...write(1, "OK",...is it really hanging or just taking longer to exit?