Wow, these are gorgeous!!
I guess they're not really really quick sketches but that isn't a bad thing I think. And I think that 15 and 10 minutes is still quite quick!
If you want to sketch more quickly it might be a good idea to draw just lines of the most important things and the main (darkest) shadow. But don't get discouraged ^^ I completely love these studies.
Just experiment with different ways of sketching and you might find that for you some are more appropriate for longer sketching and some are more appropriate for quick sketches..
P.S. haha, nice new title for your sketchbook! ^_^