Programmers are usually notoriously bad at guessing which parts of the code are the primary consumers of the resources. It is all too common for a programmer to modify a piece of code expecting to see a huge time savings and then find that it makes no difference at all because the code was rarely executed. Bentley, J. (1982). Writing efficient programs. Englewood Cliffs, NJ: Prentice-Hall., p. 32.
Bentley is author of the popular series Programming Pearls and the book Writing Efficient Programs.