The HP Z420 has an SSD storage medium which is faster than the HDD storage medium found on most desktop computers that have been around for a comparable time 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 across all desktop computers of this age and type.
Across comparable types of PC the worst storage speed is HDD.
If the storage speed of this HP desktop computer is not fast enough for you then you might also want to consider the Zotac ZBOX PI320 (Flash and higher rated), or the Asus CHROMEBIT (£99.97) (Flash and higher rated).