Not an nginx user here, but I do know sometimes certain modules in Apache will add significant latency on POST payloads by waiting for the entire POST payload to complete before sending to the backend (in a reverse proxy setup). The idea is that if the backend fails, it can retry on the next node. For large payloads this sucks for latency.. No idea if nginx has this problem.
This is a configurable option in nginx. You can have it wait until the entire request is recevied before passing to the upstream (backend) or have it stream as data arrives. I seem to recall there is still a manageable buffer when streaming too, though it's been a few years since I looked at that in detail.
