LC_ALL=C sort -k1 big{1,2,3}.txt | awk -f tally-count.awk | LC_ALL=C sort -k2nr >out.txt

real time: 12.671 seconds

LC_ALL=C parsort -k1 big{1,2,3}.txt | awk -f tally-count.awk | LC_ALL=C parsort -k2nr >out.txt

real time:  6.561 seconds

LC_ALL=C parsort -k1 big{1,2,3}.txt | ./tally-count | LC_ALL=C parsort -k2nr >out.txt

real time:  3.615 seconds