Comment by nonotmeplease

I created a 1.1G file with dictionary words and timed...

  % du -h a  
  1.1G	a
  % time look 'dog' a | wc -l
  53856

  real	0m0.021s
  user	0m0.020s
  sys	0m0.003s
  % time grep '^dog' a | wc -l
  53856

  real	0m28.593s
  user	0m0.977s
  sys	0m2.223s

Ok grep performed worse than what I expected (sort took a long time though).