As far as I know, low priority still uses bandwidth, it is just prioritized lower. So, if you streamed some video (youtube, netflix), it would prioritize the traffic from there rather than from the downloads. You are not limiting bandwidth, just telling it that if someone else comes along, to prioritize the download last. If you just download, it will still consume a lot of bandwidth. You probably want to do traffic shaping, which I dont think you can do from the gui.