Slow Down iMovie Credits Transition

You have to increase the time. It will scroll at a speed that fills the allotted time. Increase the time by dragging the left or right edge of the title to the left or right.

not to be obtuse, but when I select the 'frame' (it has yellow borders with two handles on either end), I can't move anything - I'm just trying to add credits to a slide show and having a lot of difficulty. I did try modifying the time in the properties - but that doesn't seem to affect scroll speed - I just get same speed with additional blank screen

Rather than select the frame of video, you need to select the Title which is a rectangular box above the video. You can drag the left or right edges of this box to lengthen it.

I'm also trying to figure this out. Problem is, the title cannot extend past the end. Credits are being added to the very end, not on top of any video so it is giving it one of their built-in backgrounds. I cannot figure out a way to lengthen the effect.

I cannot figure out a way to lengthen the effect.
Select the background clip and access the "cog" icon to select the "Clip Adjustments" option. Set a longer duration. Since all this does is extend the "background" you then have to select the "Text" above the clip and stretch its length to match the background clip.

I just ran into the same problem. To lengthen the clip, click on the clip in the top window, then click on the clock icon in the bottom left and type in your new clip length amount.