[in the hope of sparing people some search time]
All of the sudden, my application started feeling sluggish, and kept showing the white-yellow-red spinner consistently (my pages rely on the ProgressIndicator hack as they display a shared, authoritative, server-side timer value).
This was happening on all browsers (IE8, FF35, FF36, Chrome4).
After much hair pulling I figured out the culprit: I had just updated my anti-virus; now AVG 9.0 WebShield scans every port opening for ports 80 and 8080. Stopping the web shield brought things back to normal (phew!)
This could be construed as one more argument to provide some simple-minded server “nudge” mechanism or keep the UIDL connection open in read mode.