Why is php-fpm /php-cgi blocking concurrent requests?

Since the bug was rejected with the comment "fpm isn't maintained here ...", I did some research. Maybe this will help someone ...

The problem only occurs when a session is started with the default save handler (files). This will block requests with the same session until the first request has finished due to file locking.
To allow concurrent requests use a non-blocking save handler like memcache(d).