Photo slide show

It will depend on how you start the slideshow. There are two kinds of slideshows in Photos.

• You can show the photos from any album as an instant slideshow, when you open the album by clicking the blue text "slideshow" at the top of the album. But then there are no controls for the speed of the presentation.
• You can create a slideshow project with "File > Create > Slideshow > Apple". Then you can adjust the timing by clicking the pocket watch icon to the right of the slideshow window. It will depend on the slideshow theme, how much control you will have over the timing. Try the Classic theme for the most flexibility.

My screenshot is in German, but I hope you can identify the buttons.

As with Monterey iOS 12.5.1 at the moment, this may be entirely useless for MacOS 10.13, but here goes:

Seems in slideshows it depends on the chosen music's duration how long the individual slides show..

ie: the longer the chosen music plays, the longer the slide is viewable.

There are more controls, if you make / save the slideshow as a "project".

When you open a completed project there are a few icon / controls in the right sidebar ...

one icon looking like a clock reveals a few controls for the timing.

The Apple Support article below is for MacOS 10.14 so may be more relevant:

Create slide shows in Photos on Mac – Apple Support

Eg: If you scroll down to "Create a slide show project"

Step 8 says:

To change how long each slide appears, click the Duration button , then do any of the following (not all options are available for all themes):

• Set the slide show to match the length of the selected music: Select Fit to Music.
• Set a specific display time for the slide show: Select Custom, then drag the slider to specify how long the slide show should play.

Hope this article helps :-)

Oops ... was confused by the macOs 10.13 at the bottom of your Post. Apologies!

At the top of the linked article, you can change the MacOS to Monterey, for the accurate info.