You might try GNU parallel:

find . -type f | parallel -k -j150% -n 1000 -m grep -H -n STRING {}

( from http://www.gnu.org/software/parallel/man.html#example__parallel_grep )

You might try GNU parallel:

find . -type f | parallel -k -j150% -n 1000 -m grep -H -n STRING {}

( from http://www.gnu.org/software/parallel/man.html#example__parallel_grep )

Edit: Note that other comments that state that grep will run faster sequentially, if the bottleneck is IO, are correct.