Some more info:
Response size does not appear to be a factor. Also, TTFB (time til first byte) is over 10 sec, so server is not responding for some time.
When analyzing event time lines, I see nearly 20 timer events get fired one after the other right before an async response is received, with a combined delay of over 10 seconds. Most events are fired by jquery.flot.resize, and the others are from eventsource.js.
jquery.flot is a charting library. Since there is no chart on the page I am analyzing, there seems to be a lot of unnecessary event firing going on. This looks suspicious, so I'll target that one for my next analysis.
UPDATE::::
I disabled flot as a test and not much change, so scratch that.
Calls from Eventsource.js still take over 10 seconds when querying interfaces, so I'll look there.