Performance is one of those tough emotional issues.
This section focuses on existing storage systems and resolving performance problems, rather than on performance engineering/sizing for new systems. Resolving performance problems on systems you already own is significantly less expensive than buying new systems.
Performance problems can be just annoyances that are felt to be larger than they are due to emotional/perceptual issues. In the absence of facts, people make guesses or listen to vendors motivated by commissions. The responsible approach is to assess the real performance need (in simple, SLA-based terms) prior to throwing random money at a perceived problem.