Also true, and that's when you need to optimize your code, which sometimes means doing away with abstractions that are slowing it down, which can mean replacing elegant functional code with fugly imperative code like the OP discusses. But then after you optimize it to meet the business requirements on speed and size, it still needs to be correct.

And of course, while we're talking about business requirements, there is always project speed and budget size to account for. When optimizing for program running time and memory cost, you don't want to blow up the project's running time and dollar cost. Productivity is a huge reason why abstractions need to exist.