Your rocks are getting better .
Shinin has some good advice, don't feel trapped to use any brush or settings, play with different things. There are some fun downloadible brush settings from artists on here, or just sprinkled through the net. I've found some fun thinks to try from there.
When I first started doing the quick 30-60 second gestures we used black chalk (conte is the brand) or crayons. using something thicker lets you get down big forms like a whole leg, or an arm at once and you can focus on getting the action instead of the details of forms.
At the same time we were doing longer studies of the forms, anatomy, and things like that. Gradually the two things came together.
Speed comes with time. Don't work for speed, work for knowledge and it will come by itself