The HP Z420 has an SSD storage media which is faster than most medium processor cores personal computers that have been on the market for a similar time to it. Most comparable spec PCs that were released around the same time as it have an HDD storage media. 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 media is actually the best storage speed amongst all medium processor cores personal computers that have been around for more than two years.
Amongst similar types of PC the worst storage speed is HDD.