Extremely slow performance when a player is connected through external network

I've set up port forwarding (9000 and 3483) and use Squeeze Player for Android to connect to the server. Everything works fine when I connect using local network, the player app sees the server, and Web UI works great. When I connect by entering domain name, Squeeze Player says that it is connected to a server, but Web UI pretty much stops working. As soon as I disconnect the player Web UI unfreezes.

Logitech Media Server 7.7.5 - 1416570306 on Windows 10,