Slow down slideshow speed and add music in Photos on Mac Sequoia

To be able to control the timing of a slideshow, create a slideshow project.

Create slideshows in Photos on Mac - Apple Support (CA) (see the second section of the document: Create a slideshow project)

Create a slideshow project

1. Go to the Photos app  on your Mac.
2. Select the photos you want in your slideshow.
3. Choose File > Create > Slideshow > Photos.
4. Click the Slideshow pop-up menu and choose New Slideshow.
5. Type a slideshow name, then click OK.
6. To reorder photos, drag the thumbnails at the bottom of the window into the order you want them.
7. To select a theme, click , then click a theme.
8. You can change the theme at any time. The theme you choose determines text position, fonts, and the transitions that appear between slides.
9. To select music for the slideshow, click , click the down arrow next to Music Library, then select a song or songs.
10. Choose Theme Songs from the pop-up menu to see music included with Photos, or choose Music to see music from your library. Drag the songs you selected to change the order they play in. To delete a song, select it and press Delete.
11. Note: If you don’t see music from your music library listed, quit Photos, then open Music and sign in. With Music open, open Photos again, then select your slideshow. Click  and choose Music from the pop-up menu to see your music list appear.
12. To change how long each slide appears, click , then do any of the following (not all options are available for all themes):

• • Set the slideshow to match the length of the selected music:Select Fit to Music.
  • Set a specific display time for the slideshow: Select Custom, then drag the slider to specify how long the slideshow should play.
  • Set transitions for the entire slideshow: Select the Transition checkbox, then choose a transition type from the pop-up menu.
  • Set slides to fit the screen: Select the “Scale photos to fit screen” checkbox.

1. To see a preview of the slideshow, click the Preview button. 
2. Click Preview again to stop the preview. You can also set the slideshow to loop continuously by clicking .
3. To play the slideshow, click .

To stop playing a slideshow, press the Esc (Escape) key. Press the arrow keys to skip forward or back through the slides, and press the Space bar to pause the slideshow. 

Note: Slideshows appear under Projects in the sidebar. If you use iCloud Shared Photo Library, you must choose Both Libraries from the Library pop-up menu in the toolbar to see projects you’ve created.

To share a slideshow, see Export slideshows.