Most of the time, if you meet expectations for sequential read and write speeds, your disk subsystem is doing well. You can measure seek time, but there's little you can do to alter it besides add more disks. It's more a function of the underlying individual physical drives than something you can do anything about. Most problems you'll run into with slow disks will show up as slow read or write speeds.
Poor quality drivers for your controller can be a major source of slow performance. Usually, you'll need to connect the same drives to another controller to figure out when this is the case. For example, if you have a SATA drive that's really slow when connected to a RAID controller, but the same drive ...
