the performance of openvpn on windows can be a bit poor, but you should be able to get more than 33% of the linkspeed; if your setup is udp based then try adding
fragment 1400
mssfix
to see if it makes a difference; altering the tun-mtu size on the windows client does not do much unless the tap-win32 mtu size is also altered. You can find the optimum mtu size by running
ping -f -l <packet size> <remote-host>
For a "regular" ethernet link the largest packet you can send is 1472 bytes, which results in a MTU value of 1500 (the default).
If you find that the largest packets you can send is 1440 bytes then choose 1468 as the tun-mtu size.
