If you're trying to crack a specific password, there's no difference: you still need to hash (password + salt) for all passwords. If you're trying to crack all passwords out of a database, however, you need to hash (password + salt) for all salt. In the latter case, increasing the number of distinct salts in use increases the time-complexity of cracking, without doing anything to the time-complexity of authentication (though requiring O(n) space).