The fastest hard drives have been locked in at 15K RPM for the past few years. What has kept mechanical drives viable in performance concerned data centers is the ability to group multiple drives together in an array group. The downside to large drive count array groups is that you often run into wasted capacity because so many drives are added to hit the performance requirement that the application can’t use all the capacity that came along with the drive. Companies like 3PAR, Xiotech and Isilon get around this by performing fine grain virtualization.”]
Source: https://www.darkreading.com/database-security/understanding-hard-drive-performance