Klaus K offered the following advice in this situation.

In a crowded environment, sometimes it helps to

1. Reduce (!) Antenna power, so the access point enhances the signal,

2. Fix the data rate in order to avoid renegotiations.

sudo iwconfig wlan0 txpower 5mW
sudo iwconfig wlan0 rate 54M

You can try different values, of course.
The most important is to avoid an "auto" setting, since this triggers renegotiations.

Regards
-Klaus
I think the combination of Klaus's suggestion and Rusty's has greatly improved the
situation here. While waiting to see if this holds, I'll research Wireshark.

Thanks, Rusty.