I've got 4x8TB WD Red (I think, Red label), from WD EasyStore 8TB desktop externals that I shucked some time ago (last year, some time?).
I'm copying stuff off of the RAID-5, and it's going at ... 16-17MB/sec? WTF? What happened to my gigabit LAN. Is it the drives? Is it the QNAP NAS (*ARM dual-core CPU)? It used to run at 100MB/sec when I first got it set up. Maybe I've damaged the HDDs by bumping the NAS.
I read somewhere in the meantime that these 6/8/10TB WD Red drives, were in fact, SMR. Which might explain a few things.
Edit: My slow RAID-5 of 4x8TB and 4x10TB WD Red drives, appears to be a combination of several factors. I had enabled SMB signing, SMB encryption, "Full" SMB logging, and a few other things. Plus, my 1GbE LAN gets cut down on my main PC to around 200-300Mbit/sec when I'm mining on the CPU, so there's that too. Once I fixed all of those factors, now I'm back to copying from NAS to NAS @ 95-111MB/sec.
IOW, these drives are A-OK!