This does not help! :-/

One says that I need it and the other says I don't.
Maybe the information I provided are too lean.

We run php-fpm and nginx on the same server.
The php scripts will get the contents from different sites and sends them to the client.
We do NOT own these "different sites" (= Google, Flickr, Youtube)

@Alexandr Gomoliako:
I still don't understand what proxy_buffering really does.
When a client wants to visit Google, the php script will get the content from Google and sends it to the client.

So where is the role of proxy_buffering?
Could you provide a scenario how proxy_buffering will exactly work for our topology?

Thanks!