The Lenovo Flex 15D has an HDD storage medium which is better than the SSD storage medium found on most portable PCs of this age and type. 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.
This notebook PC's storage medium (HDD) is actually the worst storage medium across all notebook PCs of a similar age and spec to the Flex 15D.
Across similar types of notebook PC the best storage medium is SSD.
If the storage medium of this portable PC is not good enough for you then you might also want to consider the Lenovo Yoga (SSD), or the HP Pavilion x360 (SSD).