VPN Server Port Speed

I have VPN Server based on WinNT 4.0 SP6a, RRAS and MS-Proxy 2.0 with two network interfaces which are connected to internal LAN and to internet through dedicated line with enough bandwidth. When VPN Client connects to VPN Server it recieves connection speed which differ from the port speed which displayed on RRAS manager and SystemLog on VPN server (it always shows 9600 bps). How can i change the port speed settings for each VPN port on my server because it really slow for my clients even whenthere is no any activity in my internet connection?
Thanks.