Quaternion.Slerp real Speed

This is a common use of Slerp() on Unity Answers, but it is a non-traditional use. It works by walking approximately the same percentage towards the goal angle each frame. Since the starting angle is updated to the current angle each frame, the same percentage represents ever decreasing amounts of rotation as the current rotation moves towards the goal rotation. This results in an eased rotation. As for your question the greater the angle (and I assume you get a greater angle by moving the mouse faster), the greater the initial angular velocity. The only maximum velocity is when the start and end angles are 180 degrees apart, and the angular velocity drops in a non-linear fashion towards the goal.

Note if you wanted to do some testing, you could save the rotation just before the Slerp(), use Quaternion.Angle() to get the angle between the save rotation and the new rotation. Dividing the result by Time.deltaTime will give you the angular velocity for that specific frame. Note that deltaTime does not remain constant, so the velocity will jump around a bit from a perfect curve.