There needs to be more articles like this, far too often the answers on SO and the like is canned and dissimissive; "you don't need to optimize because who cares youre writing a crud app anyway".
No matter how powerful your computer and how small your computer program is, you have to optimize at least the exponential-time algorithms, and sometimes the high-degree-polynomial ones. When writing a real-time or production app you have to optimize many polynomial or even linear-time algorithms.
Micro-optimizations like allocation and boxed numbers? Unnecessary unless you need performance, only apply a constant multiplier to your program’s speed. But macro-optimizations which affect time complexity can’t be ignored.
(This isn't to say you shouldn't do algorithmic improvements, and most of the performance work I do is in fact along those lines, but I do want to clarify that the "micro-optimizations" you're talking about are in fact the difference between a computer from today and the 1990s.)