Thanks for making that observation, Ostrich. (And for backing it up with a little bit of "research" among non-ballet lovers.) It think you are right.
For non-dancers, or people not used to watching ballet much/at all, the slow motion shows up what the dancer is ACTUALLY doing. An untrained eye just isn't able to see the complexity and technical accomplishment at full speed. A friend of mine actually confirmed it when she took her non-ballet loving family to see it and they picked out those moments as most impressive.
One difficulty that non-ballet people have (and, with some choreography, I include myself among this group) is that they just can't SEE everything, even in a solo or pas de deux. If you can't really see, you can never really understand. That means that you never can become familiar with many of the movements or fully appreciate the artistry.
I'm convinced that this is one reason that audiences focus on a handful of dramatic and familiar movements that they CAN see -- fouettes, big jumps, multiple pirouettes. When the audience starts applauding these movements, you can almost feel their relief. "At last, something I can see and understand."
Like you, I also like to look at videos of certain choreography in time-lapse or slow motion, to improve my sense of what is actually going on and especially to look more closely at what the dancer is doing with his or her body. This is also why many of us who do not have the advantage of ballet training like to rewind and repeat certain parts of videos.