I'm using your X9SCL-F motherboard with disks in RAID5, the performance is really poor (7-15MB/s). When i use my disks in AHCI they can make up to 70-80MB/s. Is something wrong?
The performance of software RAID5 is poor because of the lack of cache and a proper processor. You can increase performance somewhat by turning on write-back caching, but this may also cause data loss in case of a power failure. If you need good RAID5 performace you need a hardware RAID controller.