I have set up a Linux software RAID5 on three hard drives and want to encrypt it with cryptsetup/LUKS. My tests showed that the encryption leads to a massive performance decrease that I cannot explain.
The RAID5 is able to write 187 MB/s  without encryption. With encryption on top of it, write speed is down to about 40 MB/s.
The RAID has a chunk size of 512K and a write intent bitmap. I used
-c aes-xts-plain -s 512 --align-payload=2048 as the parameters for
cryptsetup luksFormat, so the payload should be aligned to 2048 blocks of 512 bytes (i.e., 1MB).
cryptsetup luksDump shows a payload offset of 4096. So I think the alignment is correct and fits to the RAID chunk size.
The CPU is not the bottleneck, as it has hardware support for AES (aesni_intel). If I write on another drive (an SSD with LVM) that is also encrypted, I do have a write speed of 150 MB/s.
top shows that the CPU usage is indeed very low, only the RAID5 xor takes 14%.
I also tried putting a filesystem (ext4) directly on the unencrypted RAID so see if the layering is problem. The filesystem decreases the performance a little bit as expected, but by far not that much (write speed varying, but > 100 MB/s).
Disks + RAID5: good
Disks + RAID5 + ext4: good
Disks + RAID5 + encryption: bad
SSD + encryption + LVM + ext4: good
The read performance is not affected by the encryption, it is 207 MB/s without and 205 MB/s with encryption (also showing that CPU power is not the problem).
What can I do to improve the write performance of the encrypted RAID?
 All speed measurements were done with several runs of
dd if=/dev/zero of=DEV bs=100M count=100 (i.e., writing 10G in blocks of 100M).
Edit: If this helps: I'm using Ubuntu 11.04 64bit with Linux 2.6.38.
Edit2: The performance stays approximately the same if I pass a block size of 4KB, 1MB or 10MB to