"I had a folder of fifty images. I selected all the files in the folder and then duplicated them. Then I selected "Classic" for the display option. That doubled the time each image was displayed (though in reality it was showing the file twice). That will only work if a transition is chosen without any special effects. Classic is ideal. If you want a photo displayed three times as long, batch duplicate the files again. In order of appearance, you will have file#1, file#1 copy, file#1 copy copy, etc. NOTE: Do not select the "shuffle slide order" option, for you need the files to appear one after another. The transition from one slide to its duplicate is not visually detectable. It looks like simply a longer display of the one photo.The whole process can be done in half a minute if you batch duplicate files."
https://discussions.apple.com/thread/6634769?start=0&tstart=0