your ngfinx-config seems ok (except that part that should be deleted from
global-section and
appear only in event {...}
can you test your fastcgi_process with ab (apache benchmark - tool) oder
httperf until
you reach max_clients w/out reverse_proxying through nginx?
Posted at Nginx Forum: http://forum.nginx.org/read.php?2,240989,241011#msg-241011