How do I slow the transition down on Photos app?

You're welcome. 😊

Quite a few users have been asking here for an option to slow down a slideshow on the iPad or iPhone.

You may want to send feedback to Apple with a feature request. Photos for iOS - Feedback - Apple

Are you using "File>Create>Slideshow>Photos" to make your slideshow? Then the controls for timing are on the right side:

If you're using "File>Play Slideshow," then that's for quick reviews, and it's not really intended to provide any control.

You can see this:

Create slideshows in Photos on Mac - Apple Support

Are you asking about Photos on your iPad or on a Mac? You can only change the speed of the transitions on a Mac. On your iPad a slideshow will be saved as a Memory, and there it is not possible to change the duration for a single slide. The demo is fixed, way too fast. Photos in iPadOS is saving a slideshow as a memory. Personalize your memories in Photos on iPad - Apple Support (BW)

Click on the stopwatch button just below the single musical note button to bring up the timing options.

#1- you can set the length of the slideshow with this slider. Also make sure you have enough music (if you use music) to fill the time.

#2 - this lets you choose the type of transition between slides and how fast to do the transition.