I have two machines in my Wi-Fi network: a Windows machine and Ubuntu machine.
When I connect my Ubuntu computer to my Wi-Fi network, the internet speed on Windows machine becomes awfully slow. At the same time the internet speed on Ubuntu is OK. If I reboot my Ubuntu computer to Windows, all the computers in the network work fine.
I'm not running any p2p software at the time (Transmission is closed). System monitor doesn't show any significant amount of network traffic on my machine.