Slow connections over vpn for some users

I have a head office network from which we vpn to our branches. Some of our users at the branches are experincing slow issues.If that same user logs onto a different machine the same happens but if someone different logs onto both machines the speeds are quite acceptable.If the slow user uses a ms vpn client to connect to head office the speeds are acceptable and when he uses his machine in head office the speeds are acceptable.
what we have checked- adding user to admin group,tcp/dns/wins settings, lmhosts, mtu, sp2, reboot (servers/routers).
The question is why some people and not all, and why over a end to end vpn and not a client vpn?