Be careful in applying simplified models to complicated systems.
For example, Federico has been a persistent proponent for using profiling as the major (only?) guide about where to improve performance: Profiling A+B. And that is great as far as it goes. But no further.
If you were studying sharks on the decks of the fishing boat that caught them you might describe sharks as nearly blind, having no sense of smell, and unable to acquire enough oxygen by themselves. That is not a very accurate description of how they appear in their natural environment.
The point of that is that the A+B model argument applies to a system where only one program is running. The model has been simplified to the point where there is no interaction with the outside, so the A+B model is not well suited to describing the programs behaviour in a realistic system. For example, if B is very I/O intensive then it might well be the right place of concentrate performance efforts.
I/O is not the only way to hit other programs hard. Memory usage is another — especially if the program is long running.