Thanks to
@Visseroth &
@anmnz for your replies!
I was looking at the manufacturer specs for my drive model, HGST HDN726040ALE614,
https://www.hgst.com/sites/default/files/resources/DS_NAS_spec.pdf, and they claim a typical sustained transfer rate of 202 MB/s (if I'm reading that sheet correctly). So if I'm getting 144.15 MB/s write & 183.57 MB/s read speeds, I guess I'm indeed not doing that bad at all! (again, assuming I'm reading the specs sheet correctly).
SMB/CIFS speeds are a little different, though, as expected:
dd if=/dev/zero of=/Volumes/test/tmp.000 bs=2048k count=50k
51200+0 records in
51200+0 records out
107374182400 bytes transferred in 2227.664021 secs (48200349 bytes/sec)
dd if=/Volumes/test/tmp.000 of=/dev/null bs=2048k count=50k
51200+0 records in
51200+0 records out
107374182400 bytes transferred in 1550.864058 secs (69235071 bytes/sec)
Which translates into a write speed of 45.96 MB/s (approx. time of 37.11 minutes) and a read speed of 66.02 MB/s (approx. time of 25.85 minutes), out of an SMB share of that same filesystem on which I conducted my first test, accessed over 802.11ac Wifi from my 2014 MacBook Pro.
But I don't expect to be able to improve those numbers all that much until I get serious about my home networking and at least get a better router and switch. My FreeNAS rig is 10Gbps enabled, but my current router/switch is a crappy RCN-provided 1Gbps Arris DG2470A, and something tells me I shouldn't have much higher hopes for it.