Tuning I/O can help performance if a disk containing database files is operating at its capacity. However, tuning I/O cannot help performance in "CPU bound" cases, or cases in which your computer's CPUs are operating at their capacity.
It is important to tune I/O after following the memory allocation recommendations presented in Chapter 8, "Tuning Memory Allocation". Chapter 8 shows you how to allocate memory to reduce I/O to a minimum. After reaching this minimum, follow the instructions in this chapter to perform the necessary I/O as efficiently as possible.