Prime has been around for a while, and the free 2 day shipping is nice. The Prime Now app is now getting people their stuff within and hour. I could definitely see both explanations you give being believable. They aren't going to ship non-prime items as fast as the do Prime items simply because they have to justify the cost of prime to their customers. Those items are definitely at the back of the line, and they probably do intentionally slow down the process to make Prime more attractive.