How to diagnose why the ERR_CONNECTION_TIMED_OUT error appears?

Question

PHP

after transferring a self-written site to another server, sometimes when navigating through pages, the error
"ERR_CONNECTION_TIMED_OUT" occurs and after a couple of seconds the page starts loading.
web server on centos7, the site is written in php
how to understand what exactly is causing the problem?

Answer 1

View the logs and find out which requests are taking so long to complete.