The ideal ball speed is whatever gets the ball to the pocket with the right roll for optimum carry. It depends on what ball you are using, how many revs you have, your axis rotation and tilt, and the lane condition. Because of this there is no particular ball speed that is 'perfect', and it is very beneficial to be able to competently and consistently adjust your speed up and/or down at will. Ideally, find a good coach in your area, have him look at your game and assess how you are throwing the ball, and hopefully get him/her to show you how to adjust your speed consistently.