How to exorcise the devil from the webserver?

Question

PHP

There is a server, 2 sites (different domains) are spinning on it, everything works under NGINX + php-fpm
The essence of the problem:
Site 1 opens and works perfectly regardless of clients, provider, and in general any factors
Site 2 opens perfectly everywhere, except for one computer on windows 8, it loads very slowly on it, and 3 static files (two png and one js) freeze tightly, and if you open them in separate tabs, they open normally.
Test cases:
All possible caches and cookies were cleared and 3 browsers were used in the tests with a single result
1. the computer is connected to a working network (without proxies or anything) via cable or wifi - there is a problem
2. the computer is connected to a Wi-Fi distribution network from a mobile internet - no problem
3. the computer is connected to a working network and via vpn to another server in the DC - there is no problem
4. the most interesting thing: (it turned out quite by accident) as soon as they started copying a very large amount of information via rsync to another server from this server - the problem disappears for the duration of copying . If copying is stopped, the problem appears.
Tell me, what kind of academic literature to read? What type of demon? Does garlic work on it? I'm going crazy?

Answer 1

Read tcpdump, traceroute and MTU values ​​:)