Is there a way to limit the bandwidth that Software Center uses?

Whenever I am downloading packages using Ubuntu Software Center (or any package manager for that matter), my Internet connection slows to an absolute crawl. In fact, most requests timeout.

Is there a way to limit the bandwidth that Software Center uses when downloading packages?

If the software centre uses apt-get (and I'm not actually sure that it does, so you will need to try this yourself), you should be able to do the following:

Create the file /etc/apt/apt.conf.d/76download with the following content:

Acquire
{
    Queue-mode "access";
    http
    {
        Dl-Limit "150";
    };
};

That limits apt-get to 150KB/s. I haven't tried it myself, so I can't verify that it will work for you.