Ah, the reason I couldn't tell which process was the backup process is because (a) it's called "kernel task 0" which is kinda weird for a backup process, and also, it hadn't made its connection yet. When it's at the "preparing backup..." stage, it hasn't yet made a connnection, so there's nothing on the chart yet.

Once it makes a connection, the nettop command shows that it's using the ethernet connection:

kernel_task.0
tcp4 192.168.0.168:57779<->192.168.0.88:548 en10 Established

(.88 is my NAS so that's how I know)

Actually very few tasks seem to use the "en0" wlan interface, and they kind of come and go randomly. Mostly it's just a DNS responder "listen" process, and Dropbox (which I don't understand really).

Anyway, full nettop output attached, but this destroys my assumption that it's related to interface priority.

My behavior that was originally stated is still true: Wifi on, slow backup, Wifi off, fast backup. But now I have to suspect, and begin to troubleshoot, the network routing, network cabling, gig switches, or wifi router.