RST 10.1 Slow RAID 1 performance

[idata]

I added a second hard drive to my system and created a RAID 1 volume spanning the two drives. Although the system works correctly, the disk performance is now worse than it was with the single drive alone. I'm looking for hints as to what to look for that I may have misconfigured or what I have may not be appropriate for the desired RAID 1 setup.

Some basic system specs:

Dell XPS 430 with ICH9R chipset and RAID Option ROM v7.6.1.1001

Intel RST v10.1.0.1008

Windows 7 x64 Ultimate

Disk 1 - Seagate ST3750630AS (750GB, disk cache enabled, NCQ=Yes)

Disk 2 - Samsung HD103SJ (1TB, disk cache enabled, NCQ=Yes) [newly added to system]

I did disk benchmarking both before and after creating the RAID volume using ATTO Disk Benchmark (using its deafult options). All disks had approximately 100GB of data (the same data). The results were interesting (numbers are peak read/write performance):

Disk 1 alone: 75MBs/75MBs

Disk 2 alone: 137MBs/140MBs

RAID 1: 60MBs/66MBs

My expectations were that read performance should exceed that of Disk 1 (but be less than Disk 2) and write performance would be close to that of Disk 1 (probably a bit slower). I did also try enabling the Write-back Cache option on the volume. It only made a very slight increase in performance. I also double checked BIOS options and found nothing there except to confirm the drives were being managed by the RAID BIOS.

I know most (all?) people use a pair of identical drives when using RAID. Using a mismatched pair was allowed so I took a shot and seems like it could be the root problem although I don't understand why that might be true. Are there other possibilities that could account for the performanc ethat I'm seeing?