Any clues to source of this delay?

[Robin Boerdijk]

> Oh, sorry, wasn't being explicit; it isn't slow start, as the ceiling is
on
> _single_ connection, however long it stays up. By opening multiple
> connections, the server's total handling capacity goes up but single
> connections' speed stays constant.

At the risk of being completely wrong: your problem might be caused by the
fact that TCP has a sliding window of 64Kb. This means that there can never
be more than 64Kb of outstanding, unacknowledged data on a single TCP
connection. When TCP reaches that limit, it will stop sending data until an
acknowledgement from the receiver makes the window 'slide' to the right. The
larger the size of the packets you sent, the sooner the windows fills up and
the less rountrips per second you will get.

Hoping this guess is not totally off,

Robin.