Nginx and php-fpm wait to finish previous requests when 6+ concurrent connections

Question

I can't figure out how to setup Nginx and php-fpm to allow more than 6 concurrent connections from single IP. I have the following simple PHP files (no FW):

<?php
// long-request.php
sleep(15);

--

<?php
// instant-request.php
echo 'Meow World!';

If I open more than 6 browser windows with long-request.php then instant-request.php is delayed until window with first long time request is finished. When it happens php-fpm gives me the following status:

pool:                 www
process manager:      ondemand
start time:           28/Jun/2018:23:30:05 +0200
start since:          183
accepted conn:        56
listen queue:         0
max listen queue:     0
listen queue len:     0
idle processes:       0
active processes:     7
total processes:      7
max active processes: 7
max children reached: 0
slow requests:        0

My configuration: MacOS 10.13.5 (17F77), PHP 7.2.5, nginx/1.13.12

php-fpm.d/www.conf

pm = ondemand
pm.max_children = 100

I tried several configurations. I set pm to static, dynamic, I increased number of *_servers, nothing helped.

---

edit: Here is the output from /usr/local/var/log/php-fpm.log (nothing suspicious)

[28-Jun-2018 23:57:07] NOTICE: fpm is running, pid 62887
[28-Jun-2018 23:57:07] NOTICE: ready to handle connections

Answer 1

I found the answer. It was caused by the maximum number of parallel connections allowed by the browser/http protocol. Read the detailed answer at Stackoverflow.