I have a weird problem with OpenVPN for around two weeks now. I am running the OpenVPN server on a Virtual Server (German Hoster netcup) on UDP port 1194. Using this setup clients connecting through a mobile network have an extremely slow downstream (!) through the VPN tunnel (Less than 1 Mbit/s). The upstream speed is close to the general upstream capacity of the client (verified by doing a standard internet speed test without VPN). This problem doesn’t occur by connecting through a “normal” home internet connection.
If I run the OpenVPN on TCP instead of UDP everything is fine, but the general performance is slightly lower than over UDP. Additionally a lot of clients would have to change their configuration. So if possible I would like to continue using UDP. It also seems to be best practice to run an OpenVPN tunnel over UDP.
What could cause this problem? I’ve tried to play with mssfix, fragment, mtu, tun-mtu, sndbuf and rcvbuf options of OpenVPN server already- The performance got even worst. So now I am using no of these options.
I am using OpenVPN 2.4.4 on Ubuntu 18.04.4 LTS (GNU/Linux 5.3.0-53-generic x86_64) (Ubuntu Server)