Well I was going to reply to this with an amusing anecdote, then I found that the relevant chapter of Code Complete is online. Here is a sample section from this classic on benchmarking.

The anecdote was that your infinite loop reminded me of the story of a team that was trying to speed up an operating system. Well they profiled and found that it spent most of its time in one loop. So they optimized the heck out of that loop - only to find no reduction in the time spent in that code.

Turns out that they had optimized the idle loop!

Oops. :-)

On an ironic note, I have heard elsewhere that even though they did not speed up the OS, it turned out that they had made it noticably more responsive to users and smoother at multi-tasking. So it turned out to be a good thing to do after all!