ZFS has a performance problem with the zvol volumes.
Even using a ZIL you will experience low speed when writing to a zvol through the Network.
Even locally, if you format a zvol, for example with ext4, and mount locally, you will see that the speed is several times slower than the native ZFS filesystem.
zvol volumes are nice as they support snapshots and clone (from the snapshot), however too slow.
Using a pool with Spinning Drives and two SSD SLOG devices in mirror, with a 40Gbps Mellanox NIC accessing a zvol via iSCSI, with ext4, from the iSCSI Initiator, you can be copying Data at 70 MB/s, so not even saturating the 1Gbps.
The trick to speed up this consist into instead of using zvols, creating a file in the ZFS File System, and directly share it through iSCSI.
This will give 4 times more speed, so instead of 70MB/s you would get 280MB/s.