The link establishes correctly (ping etc. work) but mail and web traffic are terribly slow. I contacted Apple and they know the problem and are working on a proper fix. Their interim fix is to enter the following at the command line after you have connected.
sudo ifconfig ppp0 mtu 1492It is also suggested that you may have to make the last number lower, I have set it to 1000. I believe this is the maximum packet size.
Where does the system keep the ppp configuration information so it can be modifed once and for all? Perhaps someone can further explain what is going on.
Gavin.

