(These are pencil drawings and WordPress doesn’t scale them down too well; click on them to get a better view.)
Curiously, 5-minute sketches didn’t turn out well. That turns out to be, for me, not enough time to get things right, but just enough time to mess up the sketch.
I guess the exercise works precisely because it avoids this messing-up phase. If you don’t get the proportions right at first, it’s tempting to try to rescue the drawing by working it further– adding shading, cross-hatching, etc– and that only makes it worse. The time limit doesn’t give you the opportunity to do that, and also helps you focus on getting the important stuff down right.