Since there could be few things causing lowered performance except hardware failure so the best thing to do first is to download logs from DSS (in Web GUI -> Status -> Hardware -> Logs) and search tests.log file for:
1) hdparm - to check actual raid performance
2) ethtool - to check actual network cards speed
3) ifconfig - to see if there is any packet loss (errors in network transmission)
Please take a look at it cause it may be for example cards are running only 100MBps mode for some reason.