The Lenovo Flex 15D has an HDD storage medium which is better than most 2 in 1 laptops that came out more than two years ago. Most 2 in 1 laptops that were launched around the same time as it have an SSD storage medium. 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 medium is actually the worst storage medium across all laptops that have been around for a similar time to the Flex 15D.
Across comparable types of portable PC the best storage medium is SSD.