There was a time, long ago, when the most important thing you could do with your code was to make it more efficient — in terms of how much functionality you can pack into every kilobyte of storage, how tightly it compiles, how little RAM it uses, how much you can communicate in every network packet sent, and so on. In those days, many computers did not even have random access persistent storage, you could only run one program at a time, and RAM was measured in bytes rather than gigabytes. Those days are long gone.