I think it's got to be a configuration issue with my Windows client.
My primary router is connected to my FreeNAS through a gigabit switch. The primary router also connects to a second router through a cat5 cable. The second router is acting as an access point.
My Windows client is connected to the access point via ethernet cable, and I if I download the Ubunto torrent on this client, saving it to my FreeNAS SMB share speeds are 0.5-1mbps.
If I connect my Mac client to the access point via WiFi, I can download the Ubuntu torrent to the FreeNAS SMB share at 3-4mbps. This is still going through the same cable to the primary router, and should be slower than the Windows client if anything, owing to being a WiFi connection.
I should be able to see at least 3-4mbps on the Windows client. I'm not sure why it's much slower. I'm using qBitTorrent on the Windows client and Transmission on the Mac. It could be that Transmission is doing a better job of managing traffic to the share.