Analogy.. How fast you can speak depends on how fast your thoughts are (flow of thoughts) and.. how long have been Fast/Articulated Speaking. How fast you can type depends on Muscle Memory and.. Speed and Dexterity of fingers. Similarly, how fast you can design depends on your ability to manipulate 3d thoughts in your head and.. Familiarity with CAD software interface. First one is mental ability and second one is skill (which comes with practice) The latter can be improved within short time. But the former..