Think of it like this: you've got a line of ten people who have to review a letter before it can get mailed. You don't know it yet, but 9 of these people each take 1 day to review the letter (it can't be done any faster), and another person is quite slow and takes 10 days.
Premature optimisation: Before we even see how they work, let's train them all to halve their time! (Lots of effort spend on every person. Didn't matter for nine of them. Sort of matters for the slow person, but they only got part of the training attention.) End result: You've got a slightly faster production line, and y…