please make sure you are using Cross Hair Manager V3.12.1, not V3.12.0
and as for now, Max turning speed in Splatoon 2 is limited,
a medium-speed movement of mouse will reach it easily,
so even if you move the mouse faster,
the time drops with no faster movement on screen,
you will feel it slower than before.
Here are 2 suggestions for you:
1. move your mouse gently,
and give it some more space to move.
so it can have sufficient time to keep its movement.
2. change "Game Curve" in Cross Hair Manager to gain faster turning while your mouse movement is slow.