I used the following command to create the pool…
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sudo zpool create -O casesensitivity=insensitive Ocean raidz /dev/disk1 /dev/disk2 /dev/disk3
After some further perusing of this forum, it's occured to me that maybe I should have specified ashift=12 when creating the pool, as the 3TB WD Green are 4K drives.

I run zpool iostat during a scrub and got the following…
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marks-mac-pro:~ mark$ zpool iostat 2
capacity operations bandwidth
pool alloc free read write read write
---------- ------ ------ ------ ------ ------ ------
Ocean 5.42Ti 2.76Ti 32 24 1.33Mi 923Ki
Ocean 5.42Ti 2.76Ti 783 119 87.4Mi 893Ki
Ocean 5.42Ti 2.76Ti 420 0 49.2Mi 0
Ocean 5.42Ti 2.76Ti 1013 0 117Mi 0
Ocean 5.42Ti 2.76Ti 998 50 108Mi 216Ki
Ocean 5.42Ti 2.76Ti 818 0 83.4Mi 0
Ocean 5.42Ti 2.76Ti 721 0 70.7Mi 0
Ocean 5.42Ti 2.76Ti 575 50 66.5Mi 216Ki
Ocean 5.42Ti 2.76Ti 710 0 87.1Mi 0
Ocean 5.42Ti 2.76Ti 578 0 69.1Mi 0
Ocean 5.42Ti 2.76Ti 1.02Ki 46 123Mi 198Ki
Ocean 5.42Ti 2.76Ti 995 4 119Mi 18.0Ki
Ocean 5.42Ti 2.76Ti 1.04Ki 0 127Mi 0
Ocean 5.42Ti 2.76Ti 950 0 113Mi 0
Ocean 5.42Ti 2.76Ti 574 50 66.8Mi 216Ki
Ocean 5.42Ti 2.76Ti 962 0 108Mi 0
Ocean 5.42Ti 2.76Ti 721 0 85.7Mi 0
Ocean 5.42Ti 2.76Ti 294 50 33.8Mi 216Ki
Ocean 5.42Ti 2.76Ti 640 0 76.4Mi 0
Ocean 5.42Ti 2.76Ti 716 0 85.1Mi 0
Ocean 5.42Ti 2.76Ti 737 50 87.6Mi 216Ki
Ocean 5.42Ti 2.76Ti 1004 0 119Mi 0
Ocean 5.42Ti 2.76Ti 1.10Ki 0 134Mi 0
Ocean 5.42Ti 2.76Ti 1018 50 120Mi 216Ki
Ocean 5.42Ti 2.76Ti 1.15Ki 0 136Mi 0
Ocean 5.42Ti 2.76Ti 1020 0 114Mi 0
Ocean 5.42Ti 2.76Ti 1.03Ki 0 111Mi 0
I know that WD Greens are not the FASTEST drives in the world, but I feel I should be getting better speeds than this. I'd hate to rebuild my raid from scratch as I've already copied 3TB of files to it. I observe occasional lag from finder, such as a delay in showing folder contents, etc. Even in column view. I'm wondering if this is something to do with not specifying ashift=12 in the zpool creation. I dunno. Terminal lists the directory just fine, meanwhile Finder is still thinking about it. Maybe this is something else entirely. Either way, I still don't feel it's as fast as it could/should be.
Does anybody have any benchmarks with WD Green drives, comparing the ashift set and not set?