I haven't seen "my" answer to this problem so I am posting both Q & A here.
I am doing initial WiFi backup of 265GB on a 16" MacBook Pro, running system 10.15.1, to a 2TB Time Capsule. I have done full backups many times before: they have been slow, but not like this. Today's backup includes long stretches of no network activity, punctuated by brief – but small — bursts.
The progress bar declines to offer a prediction, but my estimate is about a week.
I have rebooted, erased the Time Capsule (via AirPort Utility, not Disk Utility) twice and re-started. Apps like Music and Photos were closed anyway. Tried the throttle hack (https://www.imore.com/how-speed-your-time-machine-backups). Pored through dozens of outdated suggestions online.
What finally got it rolling? Answer to follow.