The Flex 15D has an HDD storage media which is better than the SSD storage media found on most laptops of this type and age. HDD is a traditional storage drive which stores the information on magnetic disks. HDD drives are slower than other forms of storage such as solid state drives (SSD) and flash drives.
An HDD storage media is actually the worst storage medium across all notebook PCs of a comparable age and spec to the Flex 15D.
Across similar types of notebook PC the best storage medium is SSD.