Your motherboard does not have native SATA 3 (6Gb/s) ports. It uses an ASMedia ASM1061 controller for the 6Gb/s ports.
Current generation SSDs need to be connected to native 6Gb/s ports in order to get advertised Read/Write speeds.
1.) Update your ASMedia AHCI drivers to version 22.214.171.124
2.) Update your ASMedia controller firmware to version 3.02
See if that improves your benchmark results