Image serving via nginx are too slow, why ?

[tristanb]

Thanks for your message, 

I applyed your patched, restarted varnish, nginx and php5-fpm and it's still
the same.
Browing with browser cache off feels like the image are downloaded and
displayed in a "progressive way" because of the slowlyness.

Another example too is this PDF of 3Mo who takes >3 minutes to display :
http://goo.gl/og3xG5

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,252816,252820#msg-252820