I understand that you are inquiring about the speed difference in the transfer speeds when you have the drivers in single mode and and when running a RAID.
The RAID level 1 is used to have redundancy, this will lower the transfer speeds a lot as the hard drives check the consistence of the information.
The issue that you are experiencing is expected behavior and normal. If you are looking for speed and performance I recommend you to run a RAID level 0 but this RAID level is very delicate and if there is any data corruption, the information on both hard drives will be lost.
Another option will be to use a Solid-State Drive as cache with the Intel® Smart Response Technology to accelerate the array.