raid 1 expected performance

[michalwd1979]

Hi all,
I have PE2850 running gentoo with 2x146GB, 10k rpm hard disks in RAID 1. The raid controller is integrated perc4e/di. Can you tell me what performance should I expect from such setup? 
The HDDs are quite old, raw writing speed is about 47MB/s to each (checked it before installing RAID on normal SCSI) reading about 60MB/s. Now with RAID 1, lvm and XFS filesystem I have about 35-38MB/s writing speed. Is this normal? I assume that I will not do anything about old slow hdds but if the performance drop after installing raid 1 and filesystem is normal?

Regards,
Michael