With ZFSonLinux 0.8.4 the encryption performance is finally decent. I'd like to install a new Proxmox host - but the latest release has 'only' 0.8.3 - will this be an issue ?
a kernel with updated zfs will soon hit the pvetest repo.
You create an encrypted dataset and configure this as storage (the installer currently has no support for installing to an encrypted dataset - so the system itself will be not encyrpted) - once you boot into the new kernel you should benefit from the performance improvements