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.
I can see that this have been discussed on the forum also.
And the idea is that the closed drivers result in this effect. If you go back to the open drivers sometimes all is well again.
For my network it is very strange that only one machine slows the network down, but the amazing thing for me is that if I use the closed driver for the network nothing bad happens.
But listen, now, when I used the catalyst driver (graphics driver), the network slow down completely from 10 Mbit/s to 0.34 Mbit/s and when I shut down this machine the network gets fast again.