How to set the time between slides in slideshow (iPhoto, Mojave)

Are you referring to the time a slide is on screen or the time between switching slides. For either it's done in the "Timing" section of the slideshow mode:

Some themes will not have all of these options because they have built-in transitions.

Are you asking about iPhoto or Photos? Your question is in the forum for Photos, but you are writing (iPhoto, Mojave)?