I created an encrypted dataset in a pool created under 2.1.6 rc 4. I'm trying to copy data to it, but it is waaaaaay slow at 11 MB/sec, and the CPU percentage is up around 500%. That suggests to me that encryption is being done completely in software under this build. This machine has an Intel X5670, which is Westmere, so it should be able to use the hardware encryption. But I don't think it is. I looked through that thread we had last January/February on encryption speed; I think you had prepared a special build back then, but perhaps the changes since then have invalidated the support for hardware AES-NI encryption?
Kurt
Edit: I have a spindump of kernel_task, but it is too large to post, even after trimming it down to just the zfs-related threads.