either the game youre trying to download had an update and everyone is trying to get it like you or your internet is having a slow day. either way, try to revisit in a couple of hours and it should be fine
If your looking at you speed test and expecting to get that as your download it's not going to happen, I get 100 down at my place but my steam even set at no limit only goes as high as 15-20.
Possible that Steam is throttling you due to their server load, or network congestion outside your control, possible bad internet routing switch somewhere also beyond your control.