$ # Create two large files to work with.
$ perl -e 'printf "%08d\n", int rand 100_000_000 for 1 .. 10_000_000' > big1
$ perl -e 'printf "%08d\n", int rand 100_000_000 for 1 .. 10_000_000' > big2

# Sort them, make then unique.
$ time sort -u big1 > big1.s

real    4m0.489s
user    2m4.360s
sys     0m7.200s

$ time sort -u big2 > big2.s

real    3m24.848s
user    1m55.430s
sys     0m6.460s 

# Report the number of lines that are in the second file, and not in the first
$ time comm -13 big1.s big2.s | wc -l
8611170

real    0m14.278s
user    0m12.850s
sys     0m0.400s