I think he recognized immediately who it was.
This meal might have been the very first fast food meal in the history of the world, but fast food by our standards it was not. It is also unusual that Abraham did not even eat with them.
One of the things of course that we can learn from this and add to our fund of knowledge of the way food and eating is used in the Bible is that it is the focal point of Abraham and Sarah—the father and the mother of our faith—as their means of hospitality. It is not unimportant.