Yes, steroids fer shizzle.
Smoothness is directly proportional to speed, difficulty, and tensity. The more speed, difficulty, or tensity the stunt (and/or card artist) possesses, the more difficult it becomes to execute smoothly.
Mathematically speaking, think of smoothness like a line graph. Imagine a line representing speed, if the graph shows jagged edges, ie: /\ , it is indicative of something choppy. However, a speed CURVE would indicate a smooth execution. In a realistic approach, a card flourish starts slow for a split second and almost instantly generates speed, before the execution is complete, the flourish must SLOWLY come to a stop (for a split second it slows down to a halt as opposed to abruptly ending). Essentially, smoothness is variable speed in motion via slow, fast, slow (and other varying degrees although the example is primarily most common). To retain smoothness throughout a flourish, a constant speed helps (be it slow or fast), hiccups in the stunt brake the flowing pattern.
Often times, smoothness is an aesthetic choice, a combination of EASIER moves and/or usually two handed techniques will most definitely prove easier to do smooth. A card stunt executed with insane speed is infinitely more difficult to pull off than the same one done smoothly; however, the choice is best suited for each audience accordingly. If one is trying to impress a card artist, speed holds more value from a technical perspective whereas smoothness holds more aesthetic value - especially to the lay crowd.
Being cognizant of this concept is going to help tremendously; however, experience/practice and muscle memory will solidify in your subconscious at which point you will not have to think about any of it. Aim for balance in all of your work.
Lastly, reread this article slowly, it will help sink in a bit better.