Because it's your ISP there isn't much you can do about it.
The only thing you can try is getting an Usenet provider that offers more connections. It might be that they are throttling the speed of a single connection from your line, while they aren't throttling the overall connection speed of your line.
If it's an overall throttle they may release it if you download less over a period of time. None the less, I would call them and bitch. They'll probably swear up and down that they aren't doing it, but at least you'll feel better.
Be sure that it isn't your Usenet provider.
I've had problems with Usenet providers before like Usenethosting and PowerUsentServer not being able to provide me the service I was paying for (overselling their lines).
