The Highpoint Rocket 620 is a PCIe x1 card. It uses a Marvell 88SE9128 controller which has a maximum bandwidth of 5Gb/s (500MB/s).
The bandwidth is shared between the 2 ports on the card. Do you have 2 drives connected to the card or just the Intel 520?
PCIe x1 cards cannot handle the speeds of current generation drives. You need an x4 or x8 card to get advertised speeds from a current generation SATA 3 (6Gb/s) drive.