It's not Unraid, it's a general issue, though it Unraid in can b worse since all the devices are accessed simultaneously (if they are in the array).
Each SATA port 600MB/s is split into 5 ports, so 600/5=125MB/s max per port if all used at the same time in the same multiplier, also the JMB585 is PCIe x2 (it's only x4 physically) max usable bandwidth is around 1750MB/s, so if all drives are accessed at the same time that's around 73MB/s max per device.