It could be just about anything... Make sure you're not running any programs, if something were uploading in the background, it can severely cut your download performance, and of course if you're downloading in the background.. well that's pretty self explanatory.
If all else fails, you'll just have to contact your isp so they can check into it.
Also, some ISPs limit your dl speed if you've gone over a quota. You want to check to make sure you don't have some kind of quota that you overdid
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