I have the same problem on my 2.4GHz XP laptop connected via Wi-Fi but not on my dual-core PC with a wired Ethernet connection. The slow down occurs during the Land Grant phase. After the Land Grant is over or after I select my plot, the FPS shoots up to super speed and the game runs quickly for a few seconds to compensate, it seems.
Not really knowing anything about the game or the details of computer networking, it seems to me that it could have something to do with the Land Grant phase timer to give the quickest player the plot of land.