Decrypting only uses 2 threads.

Hi.
    I'm trying to encrypt some block devices and trying to use a
cipher and mode which is multithreaded. I'm trying out
aes-ctr-essiv:sha256 to test multithreading, but have also tried
aes-xts-essiv:sha256 with the same result.

The encryption is definitely multithread (massively), but while
reading from the encrypted block device, I see that only 2 threads are
being used. As a result when the CPU is throttled (deliberately), I
see a drop in throughput while increased CPU utilization only in 2
cores.
The following commands were used to setup the encryption --

cryptsetup -y -d - -c aes-ctr-essiv:sha256 --key-size 256 create disk1 /dev/sda

Is there anything I can do to increase concurrency while decrypting?