High response time at high concurrent connections

[bryndole]

It soulds like ab is succeeding in generating a load high enough to impact
performance already. You need to figure out where the bottlenecks are. 

My message is intended as a helpful hint that you should not trust the stats
from ab. You need to be measuring perf on the nginx and backend servers.
Start by looking at the elapsed time in nginx and the response times of the
backends.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,243246,243391#msg-243391