That's because there is something wrong with your grip, because the place where the index finger grinds the most in the straight grip should be the first joint counted from the root of the finger, not the root of the finger. It will be steady when holding the racket like this, but it will be slow when the two sides turn around, because your grip is "dead". Whether it is forehand attack, backhand push and rub, or backhand pull, the racket rests on the position of the first joint.