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User experiencing drastic slowdowns when using VPN

I am having a lot of trouble figuring out why one of my users is having so much slowdown when he VPNs into our Windows 2000 network. I have several users, including myself, who have no problems with tunneling. The only difference I can see is that he is using some kind of wireless DSL service in the Chicago area. He gets close to T1 speed when he is connected to the Internet (not VPN-connected), but as soon as he clicks on the VPN button and logs into our network in Denver, his speed slows down to about the speed of a modem. His machine is a Windows 2000 laptop, and so are all of our servers here in Denver. I keep thinking that his ISP's firewalls/routers do not comply with the VPN packets as they pass through and are dumping them, but they claim that they allow that kind of traffic. I know this is a fairly undetailed question, but do you have any suggestions that I can try with this user?
I would convince the ISP to help track this one down. There may be some sort of VPN protocol mismatch (L2TP, IPSec or PPTP). You may be somehow establishing multiple recursive VPN links (i.e., one inside another, inside another, etc.). The VPN server on the LAN may not have sufficient ports open or they may be slightly misconfigured. The wireless network may be segmenting the VPN packets too severely for reasonable throughput. You may need to adjust your IP sliding windows or VPN packet sizes. Try moving his system temporarily to a wired high-speed connection to see if the VPN gives the same throughput results.

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