The reason is how sed works: it never changes the file it is working on, but - per default - puts it results to <stdout>. The "-i" option, as ghostdog74 has pointed out, is a non-standard extension to sed and it probably works by producing an intermediary file and then replacing the original.

So the difference between what sed has to do and grep has to do is:

Code:

grep                             sed
            read the file
         parse it/change it
output to <stdout>      output to temp file
-                       replace original file with temp file

Probably you could "even the score" by having sed put its output to <stdout> too and compare the times then or - even better, as it eliminates the output delay completely - directing both grep's and sed's output to /dev/null and compare the times then.

I hope this helps.