Slide Show

While in the slideshow mode do the following:

1 - click on the stopwatch icon to bring up the timing options.

2 - select the Custom bullet and then adjust the length of the slideshow.

3 - under Trasitions you can slow down the transition speed to further slow down the show.

OR

4 - select Fit to Music and then select music track(s) to extend the length of the slideshow. More than one song can be selected.

There are two ways to make a slide show. The easiest is to select "Slideshow" at the top of an album:

But you don't have any control over the speed of the slideshow or much else.

The other way is to go to File>Create>Slideshow, and this is what Old Toad is referring to as "slideshow mode." When you create a slideshow like this, you can change the music, duration, and transition times for the images, as he shows in his insert.

It might help to look here:

Create slideshows in Photos on Mac - Apple Support