The HP Z420 has an SSD storage medium which is faster than the HDD storage medium found on most comparable spec PCs that are a similar age to it. 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 in fact the best storage speed amongst all computers of a similar age and type to the Z420.
The worst storage speed across similar types of computer is HDD.
There are a number of other SSD PCs currently available which you might want to consider including the Zotac ZBOX CA320 nano which has better reviews.
If the storage speed of this medium processor core computer is too slow for you then you might want to look at the Apple Mac Pro (Flash).