I'm experiencing the same issue. Systems on both sides are running Server 2012. System 1 has an 8 drive SATA 6Gb ZFS array, System 2 has a 12 drive SATA 3Gb ZFS array. I've benched system 1 at 1.6GB/s write speeds and I'm only getting peaks of 20MB/s sync speeds. System 2 isn't as fast but it is purely a storage server with nothing else writing to it. Both systems have 4x 1GbE Load Balanced connections using Intel Pro/1000 NIC's. As you suggested, I'm collecting the speed profile and will email it shortly. I set hdd_low_priority to false as suggested which improved peaks slightly but it still is constantly dropping down to nothing. Increased send/recv buffer size to 1000 on both sides, that didn't seem to have any effect.
