The way to make a credit roll slower is to make it last longer so it has more time to scroll up the screen. You change its length like any other event. Right click on the credit roll event on the timeline. Select Properties and then the Media tab. Then adjust the Length to be longer. The longer it is, the slower it scrolls. Finally, you have to drag the end of the event out to the tick mark to take advantage of the new length. If you go beyond the tick mark the event will loop which is not what you want.