You can’t glide for long enough to slow appreciably. And your stroke rate is slightly slower meaning fewer strokes per length, so saving energy.
My point is that the action of trying to glide that split second more results in a more relaxed stroke rate but with more efficient strokes, leading to slightly slower speed but using less energy due to fewer strokes. Which is what you were after, wasn’t it?
Seems to me (although I am a long way from an expert) that there is a linear relationship between the number of strokes/min and the energy you expend, but efficiency falls off sharply as stroke rate increases. So 20% more strokes per minute means much much harder work and not much more speed.
I could be wrong though, please correct me if you are an expert 🙂