It does not matter how proficient a developer is; the fact remains that in spite of the most meticulous attention to detail, the odds are that actual code will not run at peak performance levels around which it has been designed, at least not the first time in its generation. The actual speed of the code versus its theoretical speed tends to differ to a great degree.
This is because there are a lot of elements that combine to create a visible gap between the two. In order to substantially decrease that gap (with the core aim of ultimately closing it altogether), all code must be debugged, analyzed, and reviewed again and again until it has been determined that all of its bugs have been removed and ...
