try quickening her feet - maybe take a smaller step. She may be taking a big step, or moving her feet slowly, to allow her throwing motion to stay in rhythm. Speeding up the lower 1/2 may speed up the top 1/2.
Also look at some of Nate Trotsky's video's - in addition to the arm slots, most of his throwing demonstrations are with much smaller arm action - IIRC, he keeps the ball above the waist the whole time.
'length' is usually determined by the take away -- show her thumb to thigh, thumb to hip, thumb to chest, thumb to cheek, with proper pullback and finish actions. see if she can see that each smaller circle will give her a quicker overall throw and see if she can pick that up.
You could also try showing the girl to catch the ball off the right foot to speed up her feet when playing catch. Catch the ball off the right foot and only take one step when throwing the ball back. When done correctly, this should speed up the players feet. Please don't take this out of context, I am not saying that a player should always catch the ball of the right foot. Normally a player catches the ball of the left then takes two steps to throw or replaces their feet.