timing of photos in slide show

How are you creating the slideshow?

The instant slideshow - if you click "Play Slideshow" in an album, does not have many controls.

But a slideshow project will show the stopwatch at the right border of the Photos window.

Select the album in the sidebar, ctrl-click the album, then use the command "Create > Slideshow" from the menu, that will pop up.

Click the stop watch icon on the right border of the slideshow window to adjust the timing.

If you drag the duration slider to the right, each slide will play longer.