Open your server.conf file (see the OpenVPN installation directory in Windows and /etc/openvpn in linux) and just add these two lines:
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:
push "sndbuf 393216"
push "rcvbuf 393216"