it depends on the person. The heavier bat will obviously slow down the bat speed but the momentum you gain can give you increase pop. Depending on your stance a heavier bat can benefit you rather han a lighter one. It depends on your weight and height.

new training shows that it may slow down your bat speed. It is important to be able to swing your bat fast so it is not advisable to use a heavy bat on deck alone. If you plan to swing a big bat make sure to swing a lighter bat from your hitting bat so you don't lose the quickness of the swing.

http://www.softballperformance.com/softball-hitting-tips-improve-your-bat-speed/

Yes this is true. There was a study on this. When you swing a bat, your muscels get used to that weight and adjust to it. If you put on added weight, your muscels will tighten to adjust to the heavier weight, which causes slower bat speed due to how your muscles are tight.