Just have the same thing occurring here. Evaluating 11.1 with an areca pool. It should be easy enough to replicate - create a filesystem with compression=gzip-9 and encryption (I used aes-256-ccm). Now send 2 streams saving on that filesystem and you will notice in the prstat that the zpool process for that pool pegs at 50%. Oddly, I thought I might see something different if I turned off hyperthreading, but even after disabling it in the BIOS, 11.1 still sees 2x#cores.