Children's waterslide on the Norwegian Escape cruise ship

The thing I find interesting in this video is that the two kids going down the waterslide have such different experiences: The first kid has all sorts of trouble going down the slide... unable to get any speed up, and having to try to assist by "scooting". The second kids goes down the slide with no problems at all. Why did the two have such different experiences? Was it the lighter weight of the second child that made her go faster? Was it something about the body position of the first child that slowed things down? Did different clothing materials have anything to do with it?