Here are a few things that I would look at.
1. Networking speed, duplex, mtu, routing, port errors ect.
2. Disable MPIO from the equation use a single initiator/TPG validate access and speed it should be very quick to format and write data.
3. Verify the network you are communicating across. Ensure you are not using E0m. options interface.blocked.cifs e0M can correct this.
4. Verify the correct lun type was used and the options were setup on the volume create_ucode=on,convert_ucode=on