Xeron,
Generally speaking, RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 can suffer from poor write performance itself because parity must be updated on each write. This is because parity must be calculated on each write, requiring read-modify-write sequences for both the data block and the parity block. A discrete RAID controller will help with this because the controller processes the parity calculations. With embedded RAID, the parity calculations must be calculated using the system board processor and memory.
Embedded RAID also writes using a write-through cache setting. Write-through mode means that any cache is only used as a buffer and the drive will immediately write data the operating system instructs it to. The operating system sends a command to the drive and the operating system is forced to wait while the drive completes the write.
Regards,
John