It's a shifting target though. Let's take your example of a program with a lot of I/O. A straightforward way to optimize that is to find a way to reduce the nu

It's a shifting target though. Let's take your example of a program with a lot of I/O. A straightforward way to optimize that is to find a way to reduce the number of I/O operations you do. And once you do that, the bottleneck shifts. You're spending less time in I/O, both in an absolute sense and relative sense. So you might run into a new non-I/O new bottleneck that was just drowned out in the noise before. So you optimize that ... And sometimes this goes on for many iteration cycles and you end up with a 100-1000x performance improvement.