(Not sure why I haven't written this up yet. It sure was a problem for a long time...)
I used to work "offsite" -- outside the home -- several days a week. When offsite my laptop could perform subversion operations against a repository on my home network, with good performance. But when the notebook was "inside" the home network, those same operations would take forever.
Eventually I stumbled on the fix, clearly explained in the help sidebar of the WAN Setup page of my Netgear WGR614 router: change the MTU to 1492. (Shades of Christopher Columbus!)
Nobody reads documentation :\
The normal MTU (Maximum Transmit Unit) value for most Ethernet networks is 1500 Bytes, 1492 Bytes for PPPoE connections, or 1436 for PPTP connections.