$ time NUM_THREADS=2 ./grep-count-pcre2 "[aA].*[eE].*[iI].*[oO].*[uU]" /tmp/big.txt
parallel (2) Total Lines: 105000000, Matching Lines: 2195760

N-thds  2           5           10          20          30
real    0m9.256s    0m4.382s    0m2.748s    0m1.929s    0m1.689s
user    0m15.858s   0m16.563s   0m17.267s   0m18.391s   0m19.497s
sys     0m1.678s    0m1.339s    0m1.186s    0m1.225s    0m1.260s