In most of the templates the speed is determinded by the length of the title in your timeline, and the amount of text. Which make sense of course. 100 text entries in 5 seconds, will make the text run very fast..
Try making the title longer or shorter and see what happens.
Depending on your version (e.g.19 or 365) of PD there may be no need to download adidtional scrolling text templates, because there are some in the Credit/Scroll entry already.
Furthermore, look and see how the experts do it by searching in Youtube for PD scrolling texts. There are some good examples out there.
Have fun!