raid5 speed

I have got two machines with Areca 1882i (8 ports) cards. The machines are connected by Mellanox Connectx-3 (10Gbps fiber) cards. Both machines have got a RAID5 array (8x2TB and 8x3TB). Alle four cards (Areca en Mellanox) are on PCI-E 3.0 x8 slots. My problem is when i copy large amount of...