Why do websites freeze by domain name?

Question

Apache HTTP Server

I noticed that "friezes" begin several times a day. That is, I open sites for a long time by domain.
Freezes can be 10 minutes and suddenly completely disappear, without server reboots and other manipulations.

Any request for a domain name is processed for a long time, even if a 10-byte file is written to the server and
opened: https://xxx.net/test.txt will be a freeze.

Everyone is watching Freeze.

At the same time, there are no freezes on IP at this moment, xxx.xxx.xxx.xxx/test.txt opens without freezes (but here without https)
The server is not overloaded. Sites are running Let's Encrypt

Answer 1

Look at what specifically takes a long time - in the dev panel of Chrome, or even better with some kind of performance monitoring - webpagetest, site24x7.com, ...
Perhaps DNS servers.