Linux dm-crypt slowness with fast SSD

Question

I'm experiencing slowness with an encrypted filesystem. Here are the details:

# /dev/sdd: TOSHIBA  PX05SVQ192B      AX02  /dev/sdd

# gdisk -l /dev/sdd
GPT fdisk (gdisk) version 0.7.1

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sdd: 3750748848 sectors, 1.7 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): 51E5679B-EFA3-446F-8D55-8FAC33EB8322
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 3750748814
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048      3750748814   1.7 TiB     FD00  Linux RAID

I set up the encryption like this:

cryptsetup -y create -c aes-cbc-plain -s 256 -h ripemd160 data_crypt /dev/sdd1

Here are the test results with hdparm (slowness is also perceived also under a real load):

# hdparm -t /dev/sdd1 /dev/mapper/data_crypt

/dev/sdd1:
 Timing buffered disk reads:  2362 MB in  3.00 seconds = 787.18 MB/sec

/dev/mapper/data_crypt:
 Timing buffered disk reads:  1458 MB in  3.00 seconds = 485.34 MB/sec

I'm running kernel 3.18.7.

CPUs (2) are:

model name      : Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz

I know encryption can slow things down a bit, but this is larger than I expected. I've used cryptsetup many times without anything of this magnitude.

Can anyone offer advice on how to troubleshoot this?

Edit: I also tried doing the same thing over a loopback device in RAM:

[root@testsedbd tmp]# hdparm -t /dev/loop0 /dev/mapper/ram_crypt

/dev/loop0:
 Timing buffered disk reads:  4094 MB in  2.16 seconds = 1892.83 MB/sec

/dev/mapper/ram_crypt:
 Timing buffered disk reads:  1606 MB in  3.00 seconds = 534.89 MB/sec