I'd like to slow down the speed of roll.

Peru Bob is correct, you set the speed by the length of the graphic on the timeline.

As noted PrPro expert @Jarle Leirpoll has written in his massive PrPro tome ... to get smooth image scrolling, without jaggies, might take a small amount of math. As you want the upward pixel movement as an integer multiplier of the frame movement.

So ... you want say two or four pixels up for each frame movement, and you will get a smooth non-jaggie roll.

But at 1.5 pixels up per frame, you get ... jaggies.

You need to take your total vertical pixel movement involved, and divide by say two or four ... and that is the frame length to set your scroll to. (Or I might have that backwards ... hmmm.)

Drag it along the timeline to increase its length.