This seems obvious to me, but I've seen people, time and time again, "optimize" pieces of an application which doesn't actually have much significant impact on

This seems obvious to me, but I've seen people, time and time again, "optimize" pieces of an application which doesn't actually have much significant impact on the application, and which increases the complexity of maintenance substantially. If you can't tell me how much of an optimization a given change is, with empirical numbers (4.6% faster view code, which is 64% of the pageview, which is about a 3% overall performance increase), you don't have any business making that change. Even when you have good theoretical numbers (current approach is O(n^2), new approach is O(n)), measure it. "Bad" algorithmic complexities don't always mean "slow", and if I'm going to get 3/1000s of 1ms runtime improvement for six hours of optimization work, and you're not working on a game engine where micro-optimizations make and break you, the change isn't worth it.