Larry is exactly correct; the speed of the scrolling text is controlled by the duration. You have a fixed amount of text that needs to scroll from the bottom to the top. If you give it a short duration, it will go fast (it has less time to complete its scrolling). If you extend the duration, it will go slower.
The method of extending the duration depends on what is underneath the scrolling text. If it is a photo or if it is an added blank screen (from adding the Credits option on the Home tab), then double-click on that image in your storyboard/project area it. That will bring up the Video Tools | Edit tab where you can increase the Duration.
If you don't need the text to align to the duration of the underlying element, you can change just the duration of the text. This is done by double-clicking on the text in the storyboard/project area which will bring up the Text Tools | Format tab. From there you can change the Text Duration, independent of the clip being shown. Usually the first method is the one you want, but I thought I'd give the other method in case you need it.