Don't optimize your software prematurely
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To quote Captain Barbossa: ... the code is more what you'd call "guidelines" than actual rules.
Also, "A foolish consistency is the hobgoblin of little minds"
"Premature optimization is the root of all evil (or at least most of it) in programming." Source: Computer Programming as an Art (1974) p. 671
"In practice, it is often necessary to keep performance goals in mind when first designing software, but the programmer balances the goals of design and optimization."
^^ This. I am ashamed when the original quote is being used to basically advocate "write un-optimized code and we'll buy hardware, which is cheaper than development time". I do realize that wasn't the case back when the books were written (hardware was hella expensive), but today this is just encouraging bad form in developers, many times their go-to reaction is "let's add more ram".
Instead of discouraging the idea of writing unmaintainable code that is faster in an area that may not even be a bottleneck we get people who use that quote to excuse writing terrible code.
Good design is not premature optimization.
And it's also highly misunderstood, because it's used as a mantra, out of context.
Good development is not mantra-driven.