I have a Ubuntu desktop and a Windows 7 desktop. The Windows 7 machine is connected via wireless adapter and the Ubuntu desktop is connected via Ethernet. The wireless adapter is capable of 300Mbps link speed. When my Ubuntu is connected to the Internet, the network becomes very very slow. I have a 15 Mbps cable broadband connection. I have had many devices connected at one with no problem, yet this one connection seems to be wreaking havoc.
My guess is that there is a specific program that could be eating up your bandwidth and without actual data it is harder to debug.
There are a lot of different ways you can see what applications and services are eating bandwidth.
I would also check your router logs and see if Ubuntu is trying to use some services from your router, but being denied or generating errors.
Once you post more information I can try and debug further.