I use OpenVPN in a road-warrior setup over often slow and unreliable wireless connections. That on it’s own makes using interactive applications pretty hard.
But if you’re now additionally running OpenVPN in TCP mode over these links things get worse. The reason is, that TCP uses some kind of a three-way handshake to make sure all packets arrive in time and re-transmits those packets that don’t. With OpenVPN over TCP you now have your application’s TCP session encapsulated in your VPN'’s TCP session, doubling your ACKs and re-transmissions (if needed).
Now I switched to UDP on the VPN’s session and if the link starts to loose packets, the VPN will too, but the application’s TCP session will make sure those packets are being re-transmitted. All in all everything feels much faster - at least for a crappy 3G connection.
See this link for a more detailed explanation.