Programming in the past required finetuning the sources to get the best performance. Because you just couldn't add a 2 GHz CPU for £150 to boost instruction execution to astronomical levels.
You had a 1 MHz (C64) or a 7 MHz (Amiga) CPU and that was it! Unless you were rich and could buy a £1500 accelerator for your Amiga, which boosted the speed of a few 3D games and rendered the rest incompatible due to quirky programming techniques. (again to gain speed)