Pros are slow
Two things make it look like professional artists are fast:
Sped up process videos
Sped up videos are a much more recent phenomena and it’s obvious why it makes it appear that professional artists create artwork quickly.
Sketchy art covers anything from actual quick sketchbook studies to the loose-looking impressionistic paintings of an artist like John Berkey (one of my all-time favorites). This kind of art make it look like these virtuoso artists can just stand in front of a canvas (or at a piece of paper) and dash out a bunch of perfect strokes to create a masterpiece.
This isn’t entirely false. Pros absolutely can and do make awesome quick sketches.
But one thing I’ve noticed as I’ve looked deeper at the difference between amateurs (like myself) and professionals (or anyone who does fantastic work) is that they actually take way longer! They simply spend more time drawing and "getting it right" than amateurs. That’s it. That and tons of practice is what makes them so good.