Have you verified that all the drives are connecting at full speed ?
I have the same server, and had swapped to some new cables with latches during a server cleanup, and found that my parity check speeds were very low. It turned out that the new cables were junk. I double / triple checked that they were full seated etc, but nothing made the drives connect all full speed. I reverted to the original cables (no latch though) and link speeds were once again solid.
use dmesg|grep "SATA link" to look for the speeds, I was seeing some drives at 1.5 Gbps and some at 3.0, moving the new but obviously poor cables around moved the slow link speed to a different drive.
dmesg |grep "SATA link"
ata3: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
ata2: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
ata6: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
Surprisingly, these Monoprice cables were the problem. But nothing against Monoprice because I have used their 8087-sata forward breakout cables with 100% success on other servers.