Many people ask me how I draw so quickly, the short, and snarky, answer might be "practice", but that isn't very helpful! The long answer might include perspective, anatomy, technique etc. But, the thing that adds most to my speed when drawing is something called a "Mass Conception". Essentially, what the three dimensional form you are drawing is IN YOUR HEAD before you draw it. Usually, these forms are a nearly perfect geometric solid like a cylinder, cone, sphere, or taurus. The great draftspersons of all time use this idea.
I drew up 3 fingers that display this principle and exaggerate it so you can see it more easily. The knuckles of the fingers illustrate my point. The first finger uses a taurus and is an example of a mass conception used by the great J.C. Leyendecker. The second, a kind of flattened cube, is one of the most common used by many artists, but can be seen clearly in Frank Frazetta's work. The third is a prism- a fairly unique conception used by Michelangelo on some of his Medici sculptures. You can find many more out there and even invent your own.
The point is to actually have a shape in mind that is simple enough to draw BEFORE you draw it. Of course you need to know how to draw all of these basic forms already in perspective, and light them. Remember drawing all those spheres and cones you drew before you were allowed to do life drawing in school? Well, that's why! Most teachers have forgotten why, and the students get bored and frustrated, not knowing why they are doing what they are doing.