Most of the performance enhancements come from improved algorithms and an updated controller. The updated disks also sport new vibration control and corruption protection technology that allow the drive to adapt to any mounting problems or vibration conditions in the case, resulting in increased data accuracy, performance and reliability over time.
WD mentioned even larger performance increases to us, including a 48% boost for the 4TB drive, but we'll reserve judgment until we can test one for ourselves.
The Black's higher speeds appear to come largely from improved algorithms and an updated controller with "double the bits" of its predecessor. According to WD, the "high resolution controller" improves tracking precision, which in turn accelerates overall performance. The processor still has dual cores, but it looks like those cores have been upgraded from 32 to 64 bits—with tangible results.