Cisco VPN Client Connection Slow

I am having problems with the speed of my VPN connection. I am using Cisco VPN Client 5.0.01.0600, Windows XP and Verizon DSL. I connect quickly with no problems and I can explore folders with ease, but when I open up a file — especially a large one, it is very, very slow. However, when I use Remote Desktop connection via the VPN connection, I have no lag or speed issues. I changed my MTU to 1400, but did not see any change. Is there anything I can try? Thanks for any help!