Why does text input slow down when working in the terminal during an SSH session?

Question

SSH

Suddenly, text input in the terminal began to slow down when connecting via SSH to any servers (I tried 3 different servers located in the USA, Germany and Russia). After pressing the key, it may take 2-3 seconds until the entered character is displayed in the terminal. Tormoza are also observed when working with files via SFTP, and the file transfer speed is very low. The folder may take 10-20 seconds to open.
Empirically, I found out that the matter is definitely not in the settings of the client or server, because:
a) Inside the local network, brakes are observed on different computers under Windows and under Mac, both via a wired connection and via wi-fi.
b) When connecting to the same servers from other places, the problem is not observed.
c) If you distribute the Internet through the access point on the phone to the same computers, the braking disappears.
Accordingly, I sin on the provider, but I don’t know which way to dig further. I ran mtr and traceroute to the servers, but they did not reveal any delays.
Advise what else it is possible to check up to find the reason. Thank you.
Provider: MGTS Gpon

Answer 1

The problem was solved as follows: in the admin panel of the router, the disabled firewall switched to the "Medium" mode. After that, the brakes stopped. I can’t imagine what connection there was, but maybe my experience will be useful to someone.