When an application is slowing down because of poor storage I/O performance, the first step most IT professionals will take to solve the problem is to increase the physical drive count on the RAID group assigned to that application. Adding drives to address performance problems is no different to adding CPU’s and multiple cores to increase processing performance. The scale out model, adding more drives, has become popular because there are plenty of applications that can provide the number of applications to keep a fleet of drives busy. At that point adding drives is a case of diminishing returns.”]
Source: https://www.darkreading.com/database-security/solving-storage-performance-problems