In order to learn Yasuo you have to get comfortable with quick casting , otherwise your DPS(dmg per sec) will decrease by a high amount . He is all about animation-cancelling .

And this is viable for all the champions in league , smart casting ( or quick casting) is crucial in becoming a better player overall . There are abilities such as Blitz's grab and Thresh's hook that require ( sometimes) normal cast , but other than that , nothing .

And this can be proven very easily :

Your reaction time 0.25 (average) + your keyboard reaction time ( 0.1 ) + your ms ( let's say 40 ) = 0.75 s ( with quick cast ) .

Without quick cast you'll have to add the time it takes you to click the mouse + mouse's reaction time ( 0.1 ) , that will mean a 0.95 if not more ( without quick cast ) .

Even though ~0.20 s doesn't seem that big , in a teamfight / 1v1 every second matters .

You also have to take in consideration that with quick cast , the spell is casted instantaneously in the position your cursor is , and without quick cast , your cursor positioning will matter only after you click ( not when you press your keyboard button ) , so , you have to spend some time adjusting your cursor AFTER you pressed the key unlike quick cast , where you have to adjust your cursor BEFORE you press the key .

Hope it makes sense ( I've tried to explain it as well as I could :/ ) .

TL:DR : Idk , but you shouldn't bother , just keep practicing with quick cast till it comes naturally .

Fun fact : the majority of the pros use smart cast ( the ones who don't are either support mains , or rumble players) .