content: It might be that when you're accessing something that isn't huge such as an image file, the speed is high, but when you're accessing something that is huge, such as a movie torrent it slows down.  This is to allow "fast" internet in normal usage without actually providing huge amounts of bandwidth.  Your speedtest.net access might not be large enough to trip this.  If you download a huge file from microsoft.com or access an ubuntu installation torrent and the speed remains high then I'd guess they're throttling based on content.