But, Having Said That, ...
Stan Kelly-Bootle · Queue · 2006
A persistent rule of thumb in the programming trade is the 80/20 rule: "80 percent of the useful work is performed by 20 percent of the code." As with gas mileage, your performance statistics may vary, and given the mensurational vagaries of body parts such as thumbs, you may prefer a 90/10 partition of labor. With some of the bloated code-generating meta-frameworks floating around, cynics have suggested a 99/1 rule - if you can locate that frantic 1 percent. Whatever the ratio, the concept has proved useful in performance tuning.