I think when it comes to drawing, one of the most important (and most difficult) thing you can learn is speed.
From looking at how my favourite artists draw, i've been seeing a trend; they use lines efficiently and clearly. Let's take Kotteri's sketches for example,
Her lines are clean and confident because of her extensive experience. On the note of experience, i definetly think speed is one of the last things people achieve throughout their art journey, because it fundementally requires so much practice and fundemental of shapes. The more often you draw, the quicker it gets.
Next, efficiency does not only apply to character art, but everything elese too.
These sketches are also from Kotteri, I can tell it's her because of the curves and hatches she uses. And most importantly, the clean lines. I think training efficiency is really just training your control of lines, and that shows in everything you draw.Besides, speed gives your art rythm. I remember looking at art from 3-4 years ago and seeing how light my lines were because i was scared to make a mistake. What ends up happening is that the whole drawing looks stiff. Rythm in lines should have a blend of curves and edges in varying lengths to summarize shapes instead of having too much detail that breaks up the line of action.
Don't underestimate these sketches, they are like the foundational pillars for rendering, folds and colors. High speed also means quicker thumbnailing for bigger pieces.
Not to mention the sheer amount of Kotteri sketches i can find just on pinterest proves how much she draws