Could be incompatibility in negotiation between your new NIC and the
DSL modem. Are you sure OpenVPN plays any role at all?
Well, if I use the VPN-tunnel. speed sucks.
If I don't, speed is OK. So i'd say OpenVPN is involved in some way.
Maybe it's just the tun0-interface, but I don't know how to verify this.
Does it go away when you stop the OpenVPN tunnel?
Does it start as soon as you start the OpenVPN tunnel?
Do you lose speed for non-tunnel traffic when the tunnel is running?