The HP Z420 has an SSD storage medium which is faster than most medium processor core desktop computers that are a similar age to it. Most desktop computers that were released more than two years ago 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.
This desktop computer's storage speed (SSD) is actually the best storage speed amongst all medium processor core desktop computers that are a similar age to the Z420.
The worst storage speed amongst similar types of desktop computer is HDD.