What could be the problem with PHP5 long response?

Question

PHP

Good afternoon!
There is a powerful server: Intel Xeon 4 cores, 16 GB of RAM, 240 GB SSD drive.
Nginx 1.8
PHP 5.6
Debian 8
Static files on this server are served very quickly: 20-30ms
But there is a problem with PHP 5.6. Loading an empty file index.php 250-300 mc (Chrome), on a real site, strong delays are visible.
PHP has error logging enabled, there are minor notices that are being fixed.
The PHP-FPM (SOCK) config is as follows:
pm = dynamic
pm.max_children = 100
pm.start_servers = 12
pm.min_spare_servers = 8
pm.max_spare_servers = 16
pm.max_requests = 50
I have already tried different variations, nothing helps.
Everything is ok with static html files, apparently it's not about NGINX. restarting php5-fpm doesn't help. PHP works with Nginx via sockets.
Here is the answer of chrome, script 0kb index.php

Where to dig, how to find the error?

Answer 1

What are the cost of php extensions?
What and how is PHP installed? Is it compiled with or without zts? Is the --enable-debug option enabled in the compilation options?
Show dpkg -l | grep -i php

Answer 2

Enable php slow-logs see what's there

Answer 3

Maybe it's not a problem with the site? What about your connection? I had this recently with one site, it was also big ttfb, but now it's normal.
Check the connection from another network.