"The customer doesn't object to the app being faster, he just doesn't care. That's a big difference. If you make it faster, he won't come to you asking to slow it down because now he doesn't have time for coffee.

So, this is something that you generally ought to do, but you don't really need to."

I struggle with this personally. There are lots of things I ought to do, but don't schedule time for because they don't have a very compelling business reason to do so. The customer is happy enough to continue handing over the money at the rate that they are, and aren't likely to hand over more money if you make it faster (though a slow app could prevent new customer acquisition).