The HP Z420 has an SSD storage medium which is faster than most medium processor core personal computers that were launched more than a year ago. Most computers that have been available for around the same time as it have an HDD storage medium. Solid State Drives (SSD) store information in transistors on a circuit board rather than on optical or magnetic media. This has the advantage of being much faster than a traditional HDD drive.
An SSD storage medium is actually the best storage speed across all desktop computers of this age and spec.
The worst storage speed across comparable types of desktop computer is HDD.