poto2

Did a bit more digging, and this forum already had the answer for the Windows PC network throttling.
 
Windows nic driver setting for interrupt moderation "disabled" resolved this issue.
 
For UnRaid, what worked for me was adjusting interrupt coalescing.  My nic driver didn't allow tx-usecs adjustment, but rx-usecs increase did the trick.
 
root@unraid-one:~# ethtool -C eth0 rx-usecs 200 root@unraid-one:~# ethtool -c eth0 Coalesce parameters for eth0: Adaptive RX: n/a TX: n/a stats-block-usecs: n/a sample-interval: n/a pkt-rate-low: n/a pkt-rate-high: n/a rx-usecs: 200 rx-frames: n/a rx-usecs-irq: n/a rx-frames-irq: n/a tx-usecs: 0 tx-frames: n/a tx-usecs-irq: n/a tx-frames-irq: n/a rx-usecs-low: n/a rx-frame-low: n/a tx-usecs-low: n/a tx-frame-low: n/a rx-usecs-high: n/a rx-frame-high: n/a tx-usecs-high: n/a tx-frame-high: n/a CQE mode RX: n/a TX: n/a root@unraid-one:~# That change increased file copy speed from Win PC or TrueNAS to UnRaid cache drive quite a bit. 
 
I've a bit more reading to figure out how to make change permanent (user scripts?), and equivalent adjustment for TrueNAS, but problem solved AFAIK.
 
Thanks to @JorgeB for suggesting areas to investigate.