the / directory is the root of your file system, so before it will return it'll have to recursively open and grep every single file on your hard drive for the string 'etc'. Then it will have to sort all the lines which it finds. This will unfortunately take a while!
Try the same command in your home directory:
$ cd ~
$ grep -r some_string * | sort
Don't Stand on your head - you'll get footprints in your hair