Could it simply be that you're congested up-stream? Do you know that whomever you are sending this volume of information to, can actually take it? Do you know that the “pipe” that gets you there, can actually sustain the load?

Intuitively, I suspect that you might have far too many processes working on this, and perhaps, too many servers.

Dividing-up a computer's time does not increase it. Deploying a multiple number of processes does not accelerate any chore except to the extent that I/O overlap can be meaningfully exploited. If all of those processes are doing the same thing against the same resource, and if that resource is not subject to inherent physical speed-factors (such as the rotational and/or seek times of a disk drive), it is quite probable that they are actually getting in one another's way.

Furthermore, if this be so, then the speed-degredation will not be linear. Instead, it will reach a “thrash point,” at which point the performance graph will take a sharp knee-bend ... straight up, i.e. horribly bad. This may well be exactly what is happening to you.

(In the olden days, when disk drives were large machines that sat on the raised floor, we'd call it “going into Maytag® mode,” because the drives looked and sounded like a washing-machine on the “spin” cycle. One of them actually moved far enough to drop one leg into a hole in the raised floor. Ugh.)