How to make 2 parallel requests work at the same time in nginx?

Question

PHP

The problem is the following. There is a large online store, which was decided to be transferred from hosting to a server.
Server configured: centos7+nginx+mariadb+php-fpm.
Everything is OK, except for 1 - if we open 2 pages at the same time, which take a long time to load, then until the first one loads, the second one (or any subsequent ones (or even from another device requested at the same time as the first request)) will not load.

Answer 1

Wangyu: this is not Nginx, but in PHP you open a session and work for a long time without closing it. Therefore, the second request waits until the current session is free (in fact, until the first request ends). You need to close the session immediately.

Answer 2

BushaevDenis in the previous comment says everything correctly. See your CMS and Framework.
It can be not only sessions, but also a connection to the database.