Thanks for your reply!

I don't have performance issues. It's just the memory usage.
I don't know how nginx handles its memory, but if I increase the number of worker processes wouldn't this lead into much higher memory usage (worker_process*25GB) ?

The one worker is able to handle all requests, I don't see the point of adding more workers.
I want to know why nginx does not free the buffers after a request is finished.