Final Cut Pro (FCPX)- Scrolling credits speed
The built-in Credit Tile from FCPX does not work like that. As far as I know, the scrolling speed is determined by the duration and the amount of text. 
So if you have a long duration and few text the scrolling speed will be slow and vice versa short duration and lots of text fast scrolling. The Tile makes sure that all text is shown in the duration given. 
You might consider to try and make your own copy with Motion 5. Left Click on the Tile >> "Open a copy in Motion" or go for a 3rd party Title or Generator.