You also have to multiply time savings by the number of users. If some optimization will cost a developer an hour and that will result in only one minute saving

You also have to multiply time savings by the number of users. If some optimization will cost a developer an hour and that will result in only one minute savings for a single user, then that might seem like a loss; but only if you have less than 60 users. Once your software has a million daily users, every second you save results in a cumulative savings of 1 million seconds (over 11 days).