Original Article: https://winaero.com/speed-up-openvpn-and-get-faster-speed-over-its-channel/
OpenVPN is a well-known VPN client for secure remote access or virtual private networking. If you use OpenVPN and experience a slow speed over its channel, you might be getting annoyed. This issue is very common for all OpenVPN users. While the general advice you can find on the Internet is to tweak the MTU (Maximum transmission unit) value and/or MSSFIX parameters, here is another trick you should try. It can help you significantly improve the bandwith. Here's what you should try.
Open your server.conf file (see the OpenVPN installation directory in Windows and /etc/openvpn in linux) and just add these two lines:
sndbuf 0
rcvbuf 0
This will prevent OpenVPN from tweaking the buffer size between the server and the client. It will be determined by the OS. Windows users who connect to a Linux server will experience faster speeds.
Now, add the same lines to your client configuration file (*.ovpn or *.conf). If for some reason this is not possible, e.g. your client computer is not accessible, put the following additional lines in your server.conf file:
sndbuf 0
rcvbuf 0
push "sndbuf 393216"
push "rcvbuf 393216"