I am using Ubuntu 12.04 and my husband is using Windows 7. Ever since I installed Ubuntu, when my computer is on it slows down the entire network (when we are both connected wirelessly). We have tested the connection with both our PCs wired in to the router and with just my computer wired in to the router. When my computer is wired in, we experience no issues with the connection. I have checked the network traffic using iftop and it reports no network activity. We have confirmed the speed differences using speedtest.net. I have temporarily remedied the situation by using QOS on my router.
What is your hard drive access indicator LED doing all this time? Is it on solid, thrashing like mad? If yes, try doing this from the console prompt as your regular user:
Look for output that shows CRON activity - indicates that there are a whole bunch of CRON jobs running such as the database that holds info about programs, log rotation, etc. The jobs run right after logon as your user (typically) and you'll see quite a bit of slowdown as the cron jobs execute.
You can also try see how your connection is to a particular web site:
This will show your traffic is being managed by all the connections between your PC/laptop and the target web site.
Just a couple of ideas to play with that might shed some light on your troubles.
-*-Bill